Cockeyed Understanding Of Transitional Form

[Jackalope]

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.

They might hurt your chances for getting laid.

...or help, depending on the crowd you roll with.