Dear TJ, if you want to go outside and pretend you're Batman, go for it. (When you're old enough to engage in imaginary play and you actually know who Batman is.) I, for one, am not going to turn you in for copyright infringement. And, incidentally, no, I don't think you're wasting your time.
Every once in a while when I'm browsing online (read: can't sleep at 3am), I run across a fanfiction debate. Is it good? Bad? Legal? Immoral? And in all honestly, I don't get the big fuss.
I should confess, the second story I sold was, for all effective purposes, a Bible fanfic. (It was a short piece about Jesus as a nine-year-old.) But I've also written fanfics for stuff I couldn't sell -- contemporary books, anime, American TV, and video games. Why? Because I couldn't sell it.
Right now, I'm writing to get published. If I think something has publishing potential, I approach it slower. Post-Clarion West, I'm a lot more picky about the words I select. I don't have the massive write-first-fix-later frenzies I had pre-Clarion West. Now, I think my stories are coming out a lot better because of it. But, let's face it, writing frenzies are fun.
That's what I use fanfics for right now. Because they have no potential for publication, I can go into a writing frenzy all I want. I don't have to care all the things that slow me down when I'm writing it to be publishable. It's a totally different experience. I don't do it often, maybe once every other month, but I do it.
Do I think that aspiring writers still need to write original stuff? Well, yeah, if you want to make a career out of something, you have to practice. But I think you need to write for fun too, and if fanfics are your way of doing that, then go for it. (Exception: Your fanfic indicates a need for its author to see a mental health professional. In that case, stop writing it and go see a mental health professional.)
Besides, what exactly were we doing as kids when we were pretending to be Batman, and The Little Mermaid, and whoever else? We could've pretended to be teachers, or vets, or something we could actually aspire to when we got older. Because pretending you're Batman is fun. And sometimes, as kids and adults, we need more of that.