The principle argument against fan fiction is this, that the characters and world belong to the original author, and thus others should not deviate from this vision without his/her permission. I understand that argument, but I disagree and here's why (bearing in mind that I don't profit off it, this is just for me and any fans that I might have).
Fan fiction already exists in the mind long before it is ever put to print, just all other versions of fiction. By that point, a separate universe and version for that character already exists in the mind and consciousness, so it demands to be put to print, to be given flesh, like the Logos in the Book of John (oddly biblical for a non-Christian like me). You know that part which says, "The word (logos) was made flesh."
So, you see, trying to stop non-profit fan fic made by fans for the love of the characters is shutting the proverbial barn door after the horse left. Besides, we are all slaves to the Muse and must obey their will. Including when it comes to making fan fic.