Raven/Mystique's defection made sense to me. Her life-long friend seemed to encourage her to maintain the human form and even acted a bit visible repulsed by it at one point (the other time it was more her nudity than the form, but when she comes out of the shower in a robe in the Mystique form his reaction is hardly kind.)
So her life-long friend, and a man whom she seemed to have more the platonic interest in, wanted her to remain hidden and she even got a hint of jealousy as he was helping the others hone their abilities rather than her.
Then she falls in love with a man who understands what she goes through in having to hide who he really is -though simply wearing shoes strikes me as easier than having to use your concentration to maintain a human appearance- is very
repulsed by her natural appearance.
Then we've got this other guy, Erik. Who actually encourages
her to maintain her natural form, shows her that keeping a human form is tying her hands in her abilities.
Mystique's defection made sense to the only person between Charles, Hank, and Erik who was helping her realize her "true self" and wasn't bothered by her real appearance was was Erik. So of course she'd choose to follow him.