Why did Yusuf need to go into the field?

Given his general knowledge of multi-level dreaming and the drugs needed to do what the team needed, he could prove to be a valuable asset in the dream world should something go wrong, such as being attacked by Fischer's subconscious projections because they were unaware he'd received mental conditioning to thwart extraction methods.

They likely could have used literally anyone who was willing to be paid enough, but Yusuf already had the general knowledge they needed, meaning they didn't need to train him like they did Ariadne (remember, time was of the essence). Why risk using someone new to extraction when you can pay someone who already knows what you need to make your efforts successful?

And, as Bobby noted, they needed a driver. The first level of the dream they concoct takes place in Yusuf's mind.

First of all, I'd second MattD's answer that being acquainted with the intricacies of dream sharing, Yusuf generally made for a good companion, as they could use everyone they get.

But in addition to that, they actually did multi-level dreaming, so they might very well need Yusuf's expertise in mixing sedatives inside the dream itself. Their minds, albeit dreaming, are still bound to certain limits of physical reality in order to accept the whole dream in dream concept. For starting the 2nd level dream (and also the subsequent dreams) inside the 1st level, they still needed the dream machine along with precisely tailored sedatives inside the dream. And for this it was good to have an expert with them to mix this stuff right inside the dream (though in this case one might reason that they could have taken Yusuf even deeper with them). This is also evident from the particular dialogue of recruiting Yusuf:

Yusuf: I rarely go into the field, Mr. Cobb.

Cobb: We need you there to tailor compounds specific to our needs.

Yusuf: Which are?

Cobb: Great depth.

I had the same or similar questions concerning the plot. In the real world, the drugs and dream machines were required to get the team into the dream. However, after that, everything is imaginary: Any drugs they think they are taking, any dream machines they find in closets, any guns they have or vehicles they drive, everything is imaginary.

Now I suppose you can say that the team's subconscious minds must believe they are being given drugs in dream formulated by this master chemist guy, recognize dream machines they pull out of the closet.

However, Fisher doesn't know any of this and he must travel down with them into dreams within dreams. So his subconscious doesn't require "knowing" that a real chemist is there with them that can imagine drugs that would drive the minds deeper.

More importantly, if we accept that everyone is going into the Architect's mind and thus her dreams via technology and drugs, ok. And I suppose if we accept that they are using specialized dream within a dream machines and drugs (thus the need for a unique chemist), the team's subconscious believes they can do this once in dream. But again, Fisher doesn't.

This is one of the primary ginormous problems with the movie - the premise is that they can do what they do via these specialized machines and drugs. But these things only exist in the real world, not inside of dreams.

Furthermore, if we accept that impossible things can be done in dreams, even to the point of thinking and believing they are in dreams within dreams and their minds construct this strange reality (on top of the strange reality of what's going on in the first place) somehow out of whole cloth, why can't they do anything in the dreams AKA NEO in the Matrix movies flying around like superman?