Fantasy author Megan Lindholm/Robin Hobb comes out strongly against fanfic on her site. She tackles all the usual "pro-fanfic" arguments one by one and in great detail. Among her comments:
“I should be flattered that readers like my stories enough to want to
continue them.”
That’s not flattering. That’s insulting. Every
fan fiction I’ve read to date, based on my world or any other writer’s world,
had focused on changing the writer’s careful work to suit the foible of the fan
writer. Romances are invented, gender identities changed, fetishes indulged and
endings are altered. It’s not flattery. To me, it is the fan fiction writer
saying, “Look, the original author really screwed up the story, so I’m going to
fix it. Here is how it should have gone.” At the extreme low end of the
spectrum, fan fiction becomes personal masturbation fantasy in which the fan
reader is interacting with the writer’s character. That isn’t healthy for
anyone.
On the notion that fanfiction is good "practice" for becoming a writer, she says, in part:
No. It isn’t. If this is true, then karaoke is the path to become a singer,
coloring books produce great artists, and all great chefs have a shelf of cake
mixes. Fan fiction is a good way to avoid learning how to be a writer.
Fan fiction allows the writer to pretend to be creating a story, while using
someone else’s world, characters, and plot. Coloring Barbie’s hair green in a
coloring book is not a great act of creativity. Neither is putting lipstick on
Ken. Fan fiction does exactly those kinds of things.
Her long, self-described "rant" is worth a look.