I maintain that it’s not always so important where the characters and settings come from, but what’s done with them. And I’ve seen things done with fanfiction that go a hell of a lot further than merely playing with someone else’s toys. I’ve seen fanfics of “Daria”—an MTV spinoff from “Beavis & Butthead”—that have become taut sci-fi thrillers, ribald romances, psychological explorations, allegorical religious scenarios…almost anything you can think of, while remaining basically true to the characters, using settings the same as the original but with a few small changes, or with major changes that are yet believable as a natural evolution of the original works.
I say fanfics are like infants…you can’t condemn or deify a baby because of how it was conceived, who it’s parents were, or where it was born. You have to judge it by it’s own merits.