Well as someone who writes both serious fic and fun stuff, I tend to find the PWP and the crack!fic a fun way to 'blow off steam' from some of the weightier fic I do. And anyway, the characters are so brilliantly drawn and vivid it is easy to imagine the in all sorts of strange ways...why should Dr. Who have all the fun? :)

I dunno, I guess that is the reason for me: it's just fun.

(Me, I don't get the crossover thing. I did one (Thursday Next/LoM) due to a challenge but I don't see ever doing it again. Just seems stupid to me, but some people love the x-over thing (ghod save me from Simm!Master/Sam etc.).)