I use humans for all background vocals and I use human voices for doubling (although I use
Waves/Abbey Road Reel ADT sometimes for doubling). Although I have no objection to Melodyne cloned voices, they do not sound the same as real voices—and obviously cloning locks you into doing nothing but exactly what the lead is doing.
I do use Melodyne to tweak voices, however, because it is more efficient than multiple takes, punching in, comping, etc. (You have to be well-practiced in Melodyne, though, to make it sound natural.)
Note: I have an excellent female studio singer who can nail any weird harmony part I can write, so I am lucky.