1) Fanfiction, i think to a certain degree can be healthy for aspiring writers. I view fanfiction as a kind of writing exercise. There are plenty of creative writing prompts that will provide a little back story and characters that you are expected to extrapolate from. Fanfiction is a similar concept. You can never appreciate, as a writer, how difficult it is to command the sheer amount of creativity it takes to create a successful novel. Fanfiction is a way to cushion an aspiring writer into the world of authoring on a smaller scape. You "borrow" (this is a euphemism, dear children) someone else's brilliance and merely work in your own. In a sense, fanfiction has challenges that normal fiction doesn't, because if you are " borrowing" such beloved characters and stories then it is your responsibility to live up to the great craftsmanship of the works you've taken a free ride from.
Of course, many abuse these privileges. There should be like, rules to posting fanficiton.
1) No, you and your friends may not pose as the 10th, 11th, and 12th walkers of the fellowship.
2) No, you may not indulge in written fantasies of falling in love with any of the fellowship members.
3) Get your f*ckin' homophones right. Its/it's, their/there/they're, too/to ect.
4) Get your spelling right. I don't care if english isn't your first language or if you're just a retard. When you write crap like "He was sihlent" for silent, somewhere someone dies.
5) No, you may not marry Harry Potter
6) And no, you may not have sex with Harry Potter either.
*shudders* Horrible, horrible people. But, also, you've got to accept that many of these would-be writers are quite young, probably high school level or even jr. high level in some cases. They don't know how to write more than just the books they read or a lot of the time their attempts at writing are diluted by their illusions of good writing from television and movie scripting. Someday in college or later on, they very well may learn to write better and understand more about character achetypes and story development rather than just resulting to shoddy plagiarism. We shouldn't begrudge fanfic people of their writing development...even if it does make us want to impale someone.
2) Slash, i dunno if you mean male/male kind of slash or just the whole "I'M GONNA PAIR ALL THE CHARACTERS IN THE BOOKS IN RANDOM AND DISGUSTING PAIRINGS". I'll try and address both.
Gay relationship slash: Many fanfic writers are females*. Many women have the "wishing for a gay friend" thing. The psychology of this phenomenon in brief is that females like the idea that in a gay relationship, one of the males have to fulfill a submissive role much the same way all females face in a relationship. I'm not just talking sexually here. Women like the idea that a male would have to go through the same problems they do in a relationship. Women crave for a man to have some sort of vulnerability, which is why women go for that whole "mysterious stranger" stereotype because a man with secrets makes him vulnerable.
But anyways, the reason why people like making unlikely pairings or random pairings or going off already existing pairings is because the fanfic author or the audience that person is addressing are all craving for a wider range of emotions that the book or movie or whatever provides. In other words, they want love angst. They crave for that whole hurt/comfort blah blah blah romance kind of stuff with character they love, but perhaps cannot wholly be satisfied with because of the kind of story they're in.
Take for example, Lord of the Rings, a story now riddled by fanfiction (although some of it is quite good). In LOTR, romance was a kind of subplot. The Aragorn and Arwen thing was mainly explained in the appendix and vaguely in the story. Oh and for all those Aragorn/Legolas shippers, many were curious as to where the two had met and just how intimate of a friendship they had. Aragorn/Boromir people too were like, "DID ARAGORN KNOW HIM WHILE HE WAS STATIONED IN GOnDOR?!!!! squeee". People are just trying to fill in that hole.
*On male to female ratio on fanfic writers. I know a lot of guys, instead of fanfiction just write really bad "creative" fiction that mind as well be fanfiction. They seem to think that by changing the names of the characters and planets, they have something new and innovative instead of another rewrite of Star Wars.