Ah. Perhaps it is the British who are responsible, Emma. Now that you mention it, I can definitely hear someone like Penelope Rutledge breathlessly saying "Heaven forfend!" It sounds so...patrician.

I think the real reason people like using it is the alliteration...the two "f"'s in the word "forfend". It puts a little more of a flourish in their emphasis of just how much they don't want whatever it is that they are referring to.

On the other hand, the word "forbid" sounds stronger to my ears than "forfend". Not quite as petulant, but definitely stronger.