Anyway, as a reader my only real complaint about fanfiction is when the author's take the established story(ies) and character development and toss it out the window because it isn't what they wanted.
Actually, I don't have too much of a problem with it, as long as it's not grotesquely stupid (such as Goth!Hermione or Emo!Harry... actually, the last might be canon, I correct myself). Even then, the point is that you can't say 'only good writers are allowed to write fanfic' - because how are you supposed to pick those out, or even enforce such a silly rule? As for alternate "ships", I actually rather like some of the weirder ones. After all, it takes a certain sort of talent to write something crazy like "George/Hermione" or "Oliver/Ginny" and make it not only believable, but entertaining. And, of course, we come back to the point that JK Rowling has specifically allowed her fans to write fanfiction. I think she might have a good deal fewer fans, even, if she said no. Most fanfic writers will respect if an author says no, but they'll see it as being a little needlessly paranoid, and their respect for that author drops some. One of the bigger jokes in the fanfic community is that Anne Rice says no to fanfiction - considering all of her stuff is highly derivative, and most of it really isn't that great.
There are only two cases where it's okay to publish stuff using another author's characters: ...2. When the copyright owner gives advance permission or hires the characters out (Star Trek novels, Buffy comic books, etc.). If some fan wrote and tried to publish the untold episodes of Harry and Elaine's teenage years, or the classic love story of Thomas and Justine, then Jim Butcher would have every right to whack them on their pointy little heads with the big hammer.
I only know of a few total nutcases who have ever tried to
publish fanfiction, unless you count free web sites and archives that are explicitly there for such things. And most of those archives will refuse to web "publish" any fanfiction for authors who have explicitly stated that they don't want anyone writing fanfiction for their works. No one makes any money off of fanfiction unless they're Tom Stoppard and "Rosencrantz & Guildenstern Are Dead" is the title. If someone does try to make money off of their baby fanfic, most of the rest of the ficcers roll their eyes at them and talk about how stupid they are on their forums. Some people even email them to mention that it's not legal. You can't ask much more.
If you are using fanfic to learn in private, great! It seems one hell of a way to hone in on one part of writing, ie character or action or dialog, etc while borrowing the world created by another. You JUST DON'T have the RIGHT to ever publish, share, distribute, or even have a friend or colleage read it without permission of the original writer. It's Jim Butcher's grey matter, not yours.
There's a very big difference between stealing furniture (a common argument, which I really don't understand) and writing a derivative story. The furniture is worth something. The story is worth less than nothing, and by using it, you're not taking anything away from the author who wrote the original - in point of fact, you are advertising for them, in a sense. No one is going to read your fanfic and decide they don't need to read the original book. I have, however, read fanfiction from one of my favorite fanfic authors in a world I haven't ever heard of, and decided that I needed to get the original book from that. Howl's Moving Castle, for instance, I hadn't even heard of until someone on my f-list wrote a one-shot for it - I picked it up at the bookstore that following weekend and thoroughly enjoyed it.
In essence, there is a big difference between limiting a product and limiting a thought. You can, and should, stop people from stealing furniture. You can't stop someone from reading a book and going 'what if'? and neither can you stop them from writing a review of a book based on what they thought of it. It's a bit of an extention, but consider a fanfic a kind of positive review of a book - as in, I liked it so much that I wanted to try writing more of it.
Besides, how else do you think Harry Potter readers survive the long dry spells between books? I'd bet you a lot of money that there's a HP fanfic on quite a few of those readers' favorites lists. Dirty little secret.
Mark it up as keeping up enthusiasm between book releases - another plus for fanfiction.