Why did Borden do that to his wife in The Prestige?

Nobody would have been able to get close to the engineer (Fallon): if they had then they would have realized that he was Borden's twin. So it's not that they had to switch, it's that they both agreed to only live half a life.

Not only could they not appear in public together, when one of them was Borden the other couldn't go out as Borden, it could have raised suspicion and ruined his reputation.

So Borden got married, and when it was his twin's turn to live his life he had to keep up the pretense that he was married in order to not raise suspicion. The twin then fell in love with their assistant (Scarlett Johansson's character) but could only be with her half of the time and in secret.

So they agreed to both live half of their existence for their act, half of their time as Borden, and half as Fallon. If they would have told either of their loves, the wife or the assistant, there was the possibility that they could have revealed the deception they had kept a secret for so many years.

You are missing the point of having to switch back and forth between the two twins.

To answer your question: imagine that they need to perform the teleportation trick. One of them disappears, and the other one shows up somewhere else at the same time. The other must look exactly like the one who disappeared, wearing the same outfit, having the same props and (naturally) the same wounds/scars/ etc.

If the audience suspect for a tiny bit that the guy who appeared in a different location has a slight difference than the other one (who just disappeared), they will be on the trick and the effect will wear off quickly.

That is why they need to make sure they stay "synced and updated" to each other if they are to perform the trick at any given moment. Keep in mind that the trick has to work in reverse as well, so each of the twins must know how to talk, act, and behave as the other to avoid raising any suspicion.

Knowing all of the above, we come to your question: why involve the wife with the twin (after duplicating the injury to his hand)? The way I see it, it is nearly impossible to hide an injury from your spouse, so she has to know about it sooner or later. That's fine, we are only talking about one of the twins so far (the one who fell in love with the girl and got married to her).

Now, what we have so far, is the husband with an injured hand and an engineer with a good hand.

At this state, they can not perform the trick because one of them is different that the other (not only by looks, but also by the way an injured man moves and uses his hand).

so they must be "synced". So they do it and they duplicate the injury on the engineer's hand. Now they are equal, but wound on the engineer is fresh and needs dressing, taking care. They help each other first, but they are equal, which means, they can perform the trick again. Well, guess what: they do the trick and the "husband" disappears while the "engineer" appears somewhere else and people think he teleported. The husband must stay hidden to fool everyone, including his wife, and the engineer must take his place and goes home to fill in his space. The wife notices the wound is fresh, and possibly notices other "things" while her real husband is in the engineer disguise.

To make it short, they had to switch places because it would be difficult to keep the secret from the wife and still perform the trick in public (which includes the wife).

That is my point of view. For all I know, they could have done all of that so that we as audience can later fill in the blanks and to make it more interesting, but I agree with you, it was a stupid decision to make.

EDIT: I forgot to mention that it's been almost 8 years since I saw the movie. So when I refer in my answer to (the wife) I mean (Scarlet Johansen). I forgot if there were another love involved in the life of the twins.

Having known real twins, I think it is important to note that in addition to small physical dissimilarities (i.e. the missing pinky in the film), twins have distinct personalities.

You might be fooled in a casual meeting into thinking one was the other, but spending time with either of the ones I knew for more than a few minutes generally revealed who was who due to personality traits alone.

Why couldn't one of them just stay as Borden and the other as the engineer permanently (or switch for stage performances or special occasions, if needed)?

There would always be a period where one twin had to impersonate the other. And, as mentioned, anyone even remotely familiar with them would instantly recognize something was off.

So essentially, constant evolving practice would likely be required to fool someone personality-wise, even for a few hours (not to mention the logistical challenges of trying to keep the engineer in makeup almost all the time).

And even if they needed to periodically switch, still why involve the wife; it achieved no real purpose and potentially jeopardized the secret because she suspected something was off. If only the one that loved her had stayed with her, then everything would've been ok.

The twins are essentially mechanical magicians. Regardless of bent, they understand that tricks are a zero-sum game of knowledge, none more so than the Transported Man. For them, flawless execution of the trick is magic, both in motivational terms and in practical equivalency.

I think the movie implies a judgement call on the part of the twins that they realize this is a portion of the act that need the most work (interacting with people familiar with them).

Again, the Transported Man's success relies on zero-knowledge. Wives and girlfriends would be extremely hard to keep personality quirks from, even for short periods of time. Even a casual comment by a lover could "accidentally" give away the trick e.g. "Gee, Matilda, it seems like my husband/boyfriend is another person after the trick... crazy, right?".

I do realize that Borden was borderline crazy about his dedication to his art but still why do something that didn't add any value and almost blew his cover because his wife suspected it.

That this portion of the trick is less successful is irrelevant to the necessity of at least trying to keep up appearances.

Philosophical Stuff

• The "value" in attempting to fool the wife and girlfriend can also be seen in terms of feats of engineering. Why does Dubai have the tallest buidling in the world? Because it can.

• Angier's "The Real Transported Man" results in a shallow clone of the trick. The twins are the real Transported Man because they are attempting to become the same person (or halves of the same person) both physically and mentally. So when one twin steps out of the wardrobe, there is (ideally) no difference between himself and the twin that has disappeared. What makes a man if not both his physical and psychological countenance?

Because of his total commitment to his art

The key to understanding the whole movie and this particular part of it is revealed early during the scene when Borden and Angier are trying to work out how a Chinese magician performs his trick of making large objects appear as if from nowhere.

The chinaman appears to be a bandy-legged cripple, moving slowly and awkwardly. His trick involves magically bringing a large fish bowl into existence behind a cloth as if from nowhere. Angier and Borden observe the act and, later, observe the magician leaving the theatre. Even when not on stage the magician stays in character as a slow moving badly-legged cripple.

The dialog between Borden and Angier immediately after this observation goes a little like this:

Borden "This is the trick. This is the performance. This is why no-one can detect his methods. Total commitment to his art. Lots of self sacrifice. It's the only way to escape all this (pointing to his environment of victorian poverty)"

In the next scene Angier is trying to reproduce the trick to his wife and the dialog goes like this:

Angier's Wife: "He's been pretending to be a cripple for years"

Angier "Any time he goes out: it's unthinkable. Borden saw it at once."

The point being that this is an early hint of exactly what Borden himself has been doing to create the most prestigious trick in the whole of magic: The Transported Man. He is exploiting the fact that that he is an identical twin. And he is exploiting it by never revealing this fact to anyone, ever (just as the Chinese magician never reveals he is not a bandy-legged cripple but a strong and dexterous man). The enormous commitment to the art and personal sacrifice that this requires is a key theme of the movie, though we don't find out what Borden is actually doing until much later.

Later we find how deep this commitment goes when one twin loses fingers in an accident and the other has to replicate the injury to maintain the illusion.

The point, I think, is that he has committed to the art of this trick before he meets either Angier or his wife. Perhaps he could have let her into the secret, but that would weaken the commitment to his art and could, potentially, allow the secret to unravel. So he has to stick with the illusion despite the complications that ensue when one of the twins turns out not to love the wife the other marries. Or when the twins fail to communicate properly leading to the accident when Angier's wife is killed (and the complications that follow when one Borden can't satisfactorily explain what went wrong).

Borden's total commitment leads to tragic complications. But that is the core dramatic dilemma of the movie: stick to the total commitment to the art of the trick, or deny your life's work to avoid that cost.

Of course, in the end one Borden twin pays the ultimate cost. even then, though, he doesn't reveal the trick but is prepared to pay the ultimate price. Not that Angier does anything different as he could have saved Borden from the gallows by revealing his secret.

Both competing magicians could have picked an easier road. But then the whole dynamic of the dramatic tension driving the movie would have dissipated. The consequences of not picking the easy way is the essence of the movie and its whole point.

IMHO, it was for the 'Prestige' of doing that trick. And to keep up the 'illusion' that there was only one Borden. Borden had to show the world that he was a normal man, who had a wife and children, specially to Angier. Otherwise Angier would have suspected something was off. Borden points to the Chinese man and says "This is the illusion".

And as Angier says in the movie, "nobody cares about the man in the box". It was the magician on stage who got all the praise, and as magicians, they needed that. The Transported Man was the last trick, after which Borden had to step out into the streets, there was no time to switch back.

The only way they could keep up the 'illusion', and enjoy being the 'prestige' at the end of each show was to switch themselves at each show.

Why did Borden do that to his wife in The Prestige?

In short, because the prestige was more important to the Borden twins than the marriage.

Why couldn't one of them just stay as Borden and the other as the engineer permanently (or switch for stage performances or special occasions, if needed)?

The first part of your question can be largely addressed by Borden's smug question to Olivia regarding Angier's initial attempt to replicate "The Transported Man",

"Does he enjoy taking his bows under the stage?"

Switching roles allows each twin the opportunity to "take their bows" as well as share in the sacrifices necessary to maintain the illusion which the act requires. The two men were equal - in their dedication, sacrifice and, in their importance to each other and to the performance of their "magic".

Consider as well that the mechanics of the Transported Man require a switch. Also, Fallon's disguise is only successful because no one looks too closely at him. Fallon is in a subordinate role to "The Professor". He avoids the public scrutiny which comes with the spotlight "The Professor" revels in. Why would one twin choose all the glory and the other to live solely as Fallon?

While such pretense would present obvious difficulties in real life, the dramatic conceit positions the Borden twins in contrast to Angier. Compare the equality of importance shared by the twins to Angier's relationship with either Gerald Root (his impersonator) or his clones. In the former case of the Borden's, their personal interests are aligned. In the latter case of Angier, personal interests are at odds (invoking extortion and murder).

And even if they needed to periodically switch, still why involve the wife; it achieved no real purpose and potentially jeopardized the secret because she suspected something was off. If only the one that loved her had stayed with her, then everything would've been ok.

Though Rebecca was not privy to her role in the act, Borden's marriage was a part of the off-stage illusion. Who would suspect something so devious and cruel? Who could imagine such a dedication to art? Just like Soo's pretense as misdirection for the goldfish bowl trick, so too is the marriage further artifice distracting the audience from the mundane facts of how the Transported Man illusion is achieved.

Certainly, making Rebecca aware of the ruse could have had less of a negative impact on the marriage. Both Borden twins, however, trusted no one with the secret of their "magic" and simply placed their on-stage performance above all else. The Borden twins agreed to share their off-stage personas in support of their on-stage success. They did not trust anyone else with their secret. Though the one brother may have genuinely loved Rebecca, the brothers had already made their commitment to their art.

Why did Borden do that to his wife in The Prestige?

Apparently, "he" did not trust the women in his life to keep the secret of the trick, and valued the trick above his relationship with these women.

Everything that has already been said about the trick is correct, but the only reason the wife despaired over her husband's affair (and the assistant was forced into an illicit relationship) is because they were not let in on the trick.

An in-universe reason for this duplicity (ahem) is that the trick was only flawless if it was completely lived at all times, and never, ever discussed. No matter how loyal the women, if they had known then one or both might have wanted more than was possible within the context of the trick, or might have slipped up somewhere, or might eventually have gotten angry and intentionally spilled the beans. This would have been disastrous, as Borden's entire livelihood apparently depended on the trick.

Out-of-universe, it's easy to criticize such fanatical commitment:

• Why not live a little lower-on-the-hog and save up so he could retire sooner?
• Why not trust at least the wife? She was bound by law to be loyal, and probably had even more to lose than Borden if the secret ever came out. Even if she had to continue to live only half a life with her husband, that presumably would have been more tolerable than what she believed the state of affairs to be.
• Why not trust at least the assistant? Presumably she was in on the rest of the magician's secrets, and we know she's OK with secrets and illicit activity, at least when it comes to her one-true-love. Maybe she could even have been married off to the engineer and thereby made the secret more secure.
• Why not duck into a dressing room or water closet or phone booth à la Superman once in a while—at least when the wife was especially despondent?

If there isn't a very persuasive answer to all of these, the out-of-universe answer pretty much has to be "plot". If Borden hadn't been so very committed, if Borden's wife hadn't killed herself in despair, the whole ending of the film would have been a lot less likely and compelling. Borden's actions at the end seem to be driven as much by guilt as by a desire for revenge, and you only get to the guilt if you first have the callous treatment of the wife and the girlfriend.