Have any directors gone on record explaining why they chose to cast a famous musician in a musical film and not have them sing?

I couldn't find anything relating to Taylor Swift, but the following link refers to an interview with Idina Menzel regarding her role in Enchanted and why she didn't sing...

She said when asked:

"It's a compliment really that everybody misses my singing. Nancy was never written with a song, honestly, so I think Kevin was a fan of mine and honestly it was a compliment to be asked to just be hired on my acting talents alone."

http://www.moviesonline.ca/movienews_13514.html

Sadly, I couldn't find any interviews from Kevin Lima himself...

...However, I did find an interview at AnimatedViews.com:

http://animatedviews.com/2008/enchanted-interviews-chapter-six-kevin-lima-and-chris-chase/

And this had mentions from Chris Chase, Enchanted's executive producer, speaking about Idina Menzel and her not being cast to sing in the movie:

AV: Indina Menzel is a very famous Broadway singer in shows such as Wicked, but in Enchanted she doesn’t sing. Why’s that?

CC: We cast as an actress. That was one of the interesting things. A lot of people have asked us. They know her so well as a singer and they kind of wonder why would we cast her if her main purpose was not to sing. Kevin and I are both fans of her from Rent and from Wicked, and we’ve seen her in other things and we just thought she was a wonderful actress and she felt like the perfect New Yorker. Just like the contrast between Andalasia and, say, Times Square in New York, we wanted a contrast between Giselle, who was a princess from Andalasia and the woman who was quintessentially “New York”. So, it ultimately felt to us that she was the perfect New Yorker.

AV: In an early version of the film, didn’t she have a song?

CC: Yes. We had discussed a song that might at the end of the movie, between her and the Prince. You know what? With a different structure of the movie, it would have been a great thing. But, ultimately, the movie just couldn’t be longer. The movie couldn’t support it because she’s not the main character. It’s really not a movie about her. I would love to do another movie which is about her, because I’d love to have her in a movie where she sings! But that’s the kind of hard choice that Kevin always has to make. Sometimes, you have to let things go that are wonderful things and you want to find a place for them, but, you know, you have to stay true to the movie. The movie tells what it needs.

I think it's safe to assume there is usually one main reason why famous musicians are cast in non-singing roles (though it's something most directors would probably refrain from saying): Famous musicians can be expected to draw a certain number of viewers -- often from a demographic that the movie would not otherwise attract.

According to the interview Eliseo d'Annunzio quoted she was simply stunt casted to generate some media attention.