I can't speak as a writer, only as a reader. I feel that fic is where you go to get the stories you can't get in canon. There's no right or wrong. If a reader doesn't like it, they should go somewhere else. I used to be a staunch Doctor/Donna friendshipper, and even the idea of them kissing in a vaguely romantic way was icky. I fell in love with their canonical friendship, which continues to fascinate me. Then I read a very good romance fic, and the writer, Skyhiatrist, managed to keep everything I loved about their friendship - the banter, the closeness - and also add the romantic twist. So I think that, as long as you've got a strong enough story, or dialogue, and keep the characters in-character, writing non-canon parings can be fascinating to read.