I agree with the complexity issue. With anywhere_road where the characters hadn't met at all, zeitheist and I have basically built our own canon that sort of co-exists parallel to the show canon. That shit gets complicated, especially after the show went and killed both of our characters...

My point is that I think in many ways writing rare pairings is closer to writing original fiction and therefore the intellectual demands and challenges it places on both the writer and the reader are qualitatively different from those offered by fanfic that stays closer to canon events/pairings.

And then it takes a certain type of writer/reader who enjoys that type of challenge...