I've heard people (mostly adults) say that fanfiction is "not real writing" and "a waste of time". Now I ask you, how can it not be "real writing"? Just because you're using somebody else's characters? One of the things I love about fanfiction is the way different writers take the characters and make them their own- not make them OOC, mind you, but sort of... interpret them in their own way. Because a well-written/well-acted character has enough depth to be interpreted in many ways. Take Toby Ziegler, for example (yeah, you knew I was going to get around to this some time...) I've read countless stories featuring Toby (mostly at The National Library- best site ever go there now), and I've noticed that no two authors write him the same way. Sure, some are (in my opinion) more accurate than others, but there are some that write him totally differently from each other, but I still see both as in character (often more in character than Wells-era canon, which is why I lovelovelove Here Today/jail-centric fic). Because Toby can be interpreted so many different ways.
The X-Files is more of a problem, because (much as it pains me to say it), Scully is not as well-written/acted as Toby- her character just doesn't have as much depth, thus there is a much finer line between OOC/IC for her. (Hoo boy, now I feel guilty for betraying one of my Big Fandom Loves... *sigh*)
Another thing is, whether you're using your own characters or not, fanfiction *is* writing, and the more you practice writing, the better you get at it. Some may argue that you don't get any practice writing characters, but I would disagree (for the reasons stated above). I've read some pretty awesome fanfiction, by authors that, if they wrote original fic, could probably get published at one time or another. Writing good fanfiction, like writing good original fiction, takes time, hard work, and talent (not that I would know from experience, considering all the fanfic I write I keep inside my head).
So... yeah. Lay off fanfiction, people.
(And if I were actually more eloquent than I am, this would make more sense, and contain much less bad grammar.)