I wrote fan fic and read it for 15 years, fan fic based on TV shows. That was how I learned how to write and how I met the people I now write original material with. I also wrote a couple of fan fic pieces set in the DC Comics universe for the DC-approved Teen Titans APA (limited distribution to people on a list) back in the early-'80s). I started writing fan fic as a kid because I wanted more adventures of my fav cancelled TV shows. I got hooked on it. I could concentrate on stories and plots cuz the characters and setting already existed. Many years later, I was ready for the challenge of creating my own characters, having had a taste of that by writing the "guest" characters in fan fic.
My understanding is that it's not so much a copyright issue as copyright protects the words, not the ideas, concepts, etc. The problem is trademark infringement as the characters and name of the property usually have such protections.
Fan fic has never hurt sales or ruined box office, but rather has increased the interest and kept interest alive, ie Star Trek. In fact, ST fan fic was anthologized professionally back in the '70s in books called Star Trek: The New Voyages. A fan fic writer I know also wrote an actual Star Trek (original series) episode. Lucasfilms has been pretty okay about fan fic, too, provided characters aren't maimed, killed, or slashed. If you need to ask what slash is, you don't know much about fan fic.
What is troubling is that in the old days of print zines only, it was easy to stay under the radar and now with the web, it's all out there and the new generation of writers who post online seem oblivious to the idea that they don't own the rights to the characters they use. I attend a couple of fan cons yearly and know many of these people, cuz the same attitude has filtered into print zinedom.
BTW, a lot of pro writers (the ones I know write fantasy and SF) got their start in fan fic. I even had a story or two in zines with their stories. Jean Lorrah, who has written ST tie-ins as well as her own SF series, is the one who's been public about it so I don't mind mentioning her by name.
Okay, so there wasn't much more to post.