Cockeyed Understanding Of Transitional Form

[Angrboda]

Otherwise they died off because fully-functional bats wings wouldn't do much good to a human who was too heavy to lift off, and who otherwise would be burdened by them.

I think fully-functional bat wings would probably be a neutral mutation for a human. True, they don't have much use (other than being epically cool), but they don't exactly inhibit the human or prevent it from living normally.

I'm hesitant, given that you've included the term "fully-functional" to describe a mutation which is not functional, and certainly not functional for flight. However, the most probable mutation resulting in bat-like wings would be similar to syndactyly of the feet or hands. Not only would such a mutation be somewhat dysfunctional, it would make it extremely hard to get a date on a Saturday night.