I think one of the problems that "new" readers of fanfic have is that they read a couple of lousy stories and then they give up, thinking that is representative of fanfiction as a whole. It isn't, just as, say, one crappy book in the sci-fi genre is not indicative of the genre in its entirety. As chamekke said, If you base your fanfiction opinion on one really awful fic you find on fanfiction.net (There's a reason it's "affectionately" called The Pit) you're really doing a disservice not to yourself but also to the fabulous fanfiction writers out there. I get why people don't like it, I truly do, because it's not everyone's cup of tea. But I think it's also insulting for writers of fanfiction to be called youngsters, amateurish, etc. Many of us aren't youngsters, and we're far from amateurish. It's just that the truly awful fanfic out there has made us all look bad.
In the end, the reason why I write fanfic? I enjoy it. I love the show I write for, I love the characters I concentrate on, and I love to write. Do I plan on writing something "original," with original characters? Yes. But I don't see why I have to "give up" fanfic in order to do that, especially since it brings me a lot of joy to write.