They serve more of a purpose than bats! :-)

They are quite strong when starting the black forest. I think one of my first deaths was to one (after the swimming across a stream death and a tree to the head death), and they seem perfectly in place in the burial chambers.

I think they might be too strong to be out in the open at night in the black forest, and there's already the wraith in the swamps, like @Tangoblue said, so I don't know what else you would do with them.