I seriously do not see what the whole beef is. I know it’s because most ficcers who write OCs write shitty ones that turn into Mary Sues, and a lot of them are glamorized author insertions. That’s called BAD WRITING and it can happen in original fic too (Jane Auel, I’m lookin’ dead at YOU!). Usually, when the bad writing happens, there’s a lot of OOC going on all around. Suddenly, uber-competent canon characters are blubbering idiots while Mary saves the day and their asses. The babe magnets in canon suddenly change their whoring ways and become utterly devoted and faithful to Mary. Basically all sorts of bullshiggity goes on.
But mostly it comes from not thinking of characters as PEOPLE and not just puppets to push around on a paper or computer screen. An individual will do certain things and not do others or do things a certain way and not another way because of that individual’s makeup. An individual with a very strong code of ethics will find it very hard to violate them. Someone with loose morals will probably just go for expediency and damn the means. If you’re going to have them do otherwise, then you’ve got to set it up in the story to show WHY they’d act OOC. You can’t have a character be a mewling coward the whole story and then suddenly whoop a squadron of space troopers without showing me the transition from personality A to B.
I admit, I am biased because I write fic with OCs paired with canon characters. From the folks who’ve read them, I’ve had overall positive reviews. But then, my characters are not about to save the Enterprise or rescue the crew. Yes, I focus on the OC a lot because they are the one of the main characters of my stories.
But I also try to think them through. I ask myself questions (a lot of them) and try to answer them in the story. The main question is why would my OC and the canon character have anything to do with each other in the first place? What do they see in one another that leads them eventually to romance (because for anyone who’s ever been in a LTR, being physically attractive will get a foot in the door, but won’t guarantee a long haul and being uber perfect can be grating eventually to us mere faulty mortals)? Now the overall reason is simply because I am the author and I want them to. But within the story, inside those character’s heads, I have to answer that question and show that answer in my narrative.
But the thing in fanfic is really to each his own. If you abhor fic with OCs much less OCs that are main characters in said fic, then don’t read them (and I will continue to indicate I’m using an OC in the info section preceding my fic. Hell, I also warn for interracial pairings just in case someone has issue with that as well–and judging how my fandom reacted to Spock/Uhura, I think I’m totes justified in that apprehension). But this constant rehashing and campaigning against “teh eevulz of OCs” has gotten past tiresome.