I don't believe fan fictions are a bad thing in the truest sense of what they are. They are just a creative outlet like any other. Ray Bradbury, the Author of Fahrenheit 451, wrote fan fiction when he was a kid. He said in an interview once that he wrote a continuation of the Edgar Rice Burroughs Warlord of Mars stories, and her wrote stories about Tarzan. Fan Fiction is just a way for someone to express themselves creatively, some do it for a little while and that's all they do. Others eventually break out of writing Fan Fiction and move onto their own original work. I personally enjoy writing Fan Fiction, because when I write these stories I feel like the characters become my own. It's a very amazing feeling.
Neil Gaiman once said, "If you don't try to make things then all you've done is deprived the world of something only you could have brought to it, because the only one who has your point of view is you."
Also think about something. Geoff Johns, Alan Moore, Grant Morrison? These guys did not create Superman. Superman was created by Jerry Siegel and Joe Shuster, so essentially wouldn't that mean that the very comics we are reading today are at their very core fan fictions, based on an original writers work? Think about it.