well, I think "write what you know" is for people that want to be great artists... people that are reading fantasy romance novels aren't necessarily interested in "great art." I haven't read that many myself, but I have read a few, and people there are looking for great fantasy. Besides, it doesn't really work that well even for the really good authors... did Frank Herbert write what he knew? He wrote Dune. Not sure what he knew had to do with it. Bram Stoker wrote Dracula. Not sure what he knew had to do with it. I'm thinking that "write what you know" is one avenue to becoming a good writer; there are others.