What do you think about fan fiction?
And yes, I know there are stories about big-time comic book writers getting gigs based on fan fiction (I personally know one of them, and her lucky break, she'll admit, was not the standard way inside), but that's a one-in-a-million nowadays, and every time somebody finds a way in other than the old standby of building a portfolio of work that gets you eventually noticed, trust me, someone finds a way to plug that hole so others can't slip in that way again. *grins*
The difference is that when a publisher with the rights to using a character (even if said rights come from public domain usage) is paying you to write a story based on that character, that's not fan fiction even if you happen to be a fan of the character. You have to follow the rules that fan fiction can ignore (He-Man vs. Strawberry Shortcake, anyone?) and you have to answer to your corporate bosses.
For example, me writing a Superman story in order to have Clark fall in love with Buffy while the two of them save Harry Potter from a rabid Pokemon is clearly a fan fic, but me being paid by DC to write a Superman story for their continuity is work for hire and is adding to the core story of that character that I don't happen to own. It's like the work I'm doing for Pulp Obscura now. Sure, it plays on the same emotions that a fan fic featuring some of the old public domain characters would, but what I'm doing is adding to the mythos surrounding the characters based on the publisher's interest in building that new, ongoing story.
So, I guess for me it boils down to this: If you want to write fan fiction for your own enjoyment or for the enjoyment of those people who dig the cultural phenomenon that is the world of fan fic sites, knock yourself out. But if you want to work in the industry, leave the fan fiction behind and pursue a portfolio that shows off your stories that you've worked hard to get published in the real world (so to speak).