I know, old thread, but I just wanted to put my two cents in on the original question.
I usually don't have a problem with fanfic. What I hate about it is when someone comes along and takes a pre-created story and characters and starts to bring back to life dead characters, changing the themes, and heavily altering the personalities. That's what makes fanfic, good fanfic, hard. In a given story a character is supposed to go through character development. It's almost impossible to do this without changing the pre-created characters to the point of being unrecognizable to the original. (The biggest reason I’m chiming in on this is because my favorite book series of all time had a very unpopular ending and kids—yes, it is a children’s book, but not Harry Potter—everywhere started making fanfic with the only casualty in the entire series coming back to life--Note: the story's theme was "war and how it changes people". Stupid kids.)
I also don’t think this helps writers in the long run. Learning how to create life-like and likable characters is probably the biggest part of writing, IMO, and fanfic isn’t doing that when you're just recycling old characters. I write speculative fiction, and in it you have to learn to world build, that is another thing that you can’t go wholesale on with a fanfic.
Yes, it is fun (I’m assuming, I’ve never written any), and it may be good practices with learning how to plot and use dialog, but it’s not going to improve one’s writing like original fiction can.
