Yes, the "fanfic is derivative and therefore inferior!" argument has always struck me as completely ridiculous because, first off, there's nothing written in any genre that doesn't draw upon the work that's come before, and if that wasn't enough, I think anyone can come up with dozens of examples of "pro" fanfic -- some of which are noted in my comment below, but seriously, I think anyone who reads has encountered at least one and probably a lot of books that are based upon famous novels, plays, myths, religious texts or historical figures.