Their theology alone would make them off-limits to the vast majority of trinitarian Christians.
I don't have any way of proving this, but I get the impression that Mormonism pulls most of its converts from people who were somewhat estranged from mainstream churches to begin with. It's hard to imagine a devout and content Baptist, for example, thinking he should explore further the idea that Jesus was the product of divine sex on another planet.
So, if America undergoes a series of cataclysmic events that utterly destroy traditional Christianity, like, say, Red Dawn happens and the commies win, maybe when it's all over and things are getting back to normal, the Mormons can be the only faith left that's capable of drawing in large numbers.