I’ve had the chance to observe a few pro level groups in the studio, and I have yet to see a singer look at a piece of arranged sheet music.
I do know of one such session, though. When Mosie Lister produced a CD for the Dove Brothers a few years ago, he wrote out all the vocal parts in advance. McCray said it took a while to learn them.
I agree with FamilyMan on the point of creativity. I’ve been around some groups that brag about how fast they can do a vocal session. If you listen to the music those groups turn out, it may be good in terms of tone, blend, etc., but it’s the usual run-of-the-mill in terms of the overall sound.