Possibly what I'm about to say has been beaten to death, I'm not sure because I haven't read every single comment. But anyway. Though feedback is nice, it's not what I write for. I think the problem is that the rarepairs are less visible, recced less often because fewer people like them, and thus read less often, which translates into less feedback. But I'm among those who do their damnedest to leave some sort of feedback for everything I read, and I know that's not the case with everyone.
I'm *still* getting reviews from the horrible Harry/OOC!Draco stuff I wrote two years ago. If I wrote for the reviews, I suppose I'd go finish that stuff. I started writing Snape/Harry just at the tail end of when it was still a rarepair, though, and I actually get less feedback for it now than I did then. Possibly because I'm not the world's greatest writer, and with the popularity of the pairing there are better authors than me writing it.

Er. Anyway. I think I'm rambling, and I don't have a real conclusion to draw from any of this, so I'll shut up. :)