I feel like in the end, it just comes down to "I can't feel good about myself and/or my place in fandom/the world unless I've made someone else feel bad, or at least made them look bad to someone/everyone else". I don't think every expression of negativity falls under this rubric, obviously (ha!): I consider negativity that (even potentially) leads to dialogue as important, even critical, to online discourse (sorry for the bad pun). But whereas "this fic blows for reasons XYZ" or "I dislike slash because its creators often undeservedly pretend to be a force for social change", while certainly inflammatory, can open a discussion, "you write RPS/engage in cosplay/draw cross-gen fanart, and therefore you're weird/creepy/fail as a human being" can do no such thing. It can only hurt or offend, and its only benefit is to the speaker (and any listeners who really dislike the target or feel similarly judgmental about her brand of fannish activity).
When non-fen see fannish pursuits as weird or creepy, I don't mind it. They are not engaged in fandom, so it's practically impossible for them to see it in context -- what's mainstream to many fannish folk is "omfgz Snape diddling Harry how awful and gross!1" to the average middle American hausfrau. Hearing "omigosh, what a freak show!" from one harried businessman to another while trying to cross First Ave in San Diego during Comic Con doesn't make me want to throttle the speaker. I don't think outsiders owe it to us to understand and accept fandom in all its bewildering majesty. We're not a marginalised group; we're engaged in a hobby. It's a frivolous leisure-time pursuit you can walk away from without serious (i.e. life-threatening or life-defining) consequences. I think cries of "we the disenfranchised" from fen (vs non-fen), at best, are tacky. At worst, they're potentially offensive to the truly disenfranchised.
If you have a weird hobby, other people might look at you funny. I think that's okay, as long as they don't actually hurt you -- it's impolite, but if nothing else, a tendency to regard unfamiliar things with at least some suspicion has a very distinct evolutionary benefit. But when people WITH THE SAME HOBBY look at you funny because you're not engaging in the hobby in ways they approve of? I am firmly of the opinion that those people are blithering morons who need a boot to the head. Or