Personally, I have no interest in fanfiction, either in reading or writing it. There's plenty to read that's already on the shelves (and has already passed QA, so to speak); and as a writer, I'd rather be getting stuff done.
Fanfiction as a "thing", however, I don't really see as good or bad; it's just something that's not for everybody.
I think it's an exaggeration to say that fanfic is "vital" or "essential" part of learning to write. (At least, I should hope so; I've never written any.) But I also think it's incorrect to say that it won't teach you how to write, or how to show characterization, or to world-build.
Sure, you may be using someone else's world or characters, but showing characterization and world building and stuff like that is about execution at least as much as it is about actual invention. (More, even, but that's an argument for another time.) You can still learn to show world-building and characterization while writing someone else's characters and someone else's world. And you can learn other valuable things, such as story construction and even just how to write prose.
So yeah, if one's doing it as a way to hone one's writing craft, I think it's a viable approach, even if it's not stuff you can publish.
And not all people who write fanfiction are doing it with that in mind. They're doing it because they enjoy writing, or enjoy the world, or both. They're not necessarily looking to get paid or published. And that's fine.
But yeah, if I were going to write fanfiction, I'd totally be trying to get on board with Wizards. XP
