ohnotheydidnt has a thread about what famous writers have to say about fan fiction. It is about as horrid as you imagine, with Orson Scott Card 'rigorously defending his copyright', Ursula LeGuin likening it to colonization and JK Rowling being awesome and chill as she always is. I think Joss Whedon said it best: "All worthy work is open to interpretations the author did not intend. Art isn't your pet -- it's your kid. It grows up and talks back to you." Art is inherently collaborative (as all forms of communication are). You can't "own" a fictive commodity. It goes against the very nature of what art should be about--openness, sharing and community. Reworking familiar plots and stories is as old as writing itself.
And fuck you sideways if you're one of those authors (ANNE FUCKING RICE) who uses threats and intimidation to force takedowns of fan-derived works, which we lovingly create and make no money from. You should be happy that people read your stupid books (GEORGE R. R. MARTIN) and care enough to slash Ryrion/Jon Snow (or whatever the hell, I don't really like 'Game of Thrones'). Fan fiction does not affect your bottom line. It is not identity theft (ROBIN HOBB). It isn't psuedo-writing...as Neil Gaiman put it,
I think that all writing is useful for honing writing skills. I think you get better as a writer by writing, and whether that means that you're writing a singularly deep and moving novel about the pain or pleasure of modern existence or you're writing Smeagol-Gollum slash you're still putting one damn word after another and learning as a writer.
The truth is that copyright is a relatively new development, and a shitty one as far as I'm concerned. As a person who does the actual intellectual work that copyright was meant to protect (ie, research at land-grant university, rather than money-making storefront bookselling), I can tell you that people who do the work I do acknowledge their indebtedness....but continue to re-work and use ideas from other people. That is the way that intellectual work functions and without that sort of collaboration, it dies. Fan fic teaches the same thing that I teach university students...that knowledge is an ongoing conversation, mediated by the voices of many people, and as long as we continue to acknowledge their voices and to be ethical towards the originators of the idea, we are doing no wrong in talking through and adding on to their work.
Now if you'll excuse me, I have a Thor/Loki fan fic to read...