The alien bacteria thing always bugged me, too. "Now you've just moved the question of how life began out to an asteroid! That doesn't help at all!" It's actually the same thing as saying that "The universe is so complex that it MUST have been designed by someone!"--which always brought me straight up against "Yeah, but by your own logic, the designer is too complex to have just appeared, right?" It's just moving the problem over.

Although some of the studies of alien life do suggest they're another possible way to extrapolate the existence of life in lots of places, which is cool, but we'll see ...