Author and blogger Cory Doctorow has an excellent article in defense of fan fiction and the people who write it:
Many pros got their start with fanfic (and many of them still work at it in secret), and many fan-fic writers are happy to scratch their itch by working only with others’ universes, for the sheer joy of it. Some fanfic is great — there’s plenty of Buffy fanfic that trumps the official, licensed tie-in novels — and some is purely dreadful.
Two things are sure about all fanfic, though: first, that people who write and read fanfic are already avid readers of writers whose work they’re paying homage to; and second, that the people who write and read fanfic derive fantastic satisfaction from their labors. This is great news for writers.
Great because fans who are so bought into your fiction that they’ll make it their own are fans forever, fans who’ll evangelize your work to their friends, fans who’ll seek out your work however you publish it.
Our field is incredibly privileged to have such an active fanfic writing practice. Let’s stop treating them like thieves and start treating them like honored guests at a table that we laid just for them.