Seems like this thread was fishing for a flame war or something. Just by posing your question...no I won't even open that can of worms. :P Sorry.
Personally never heard of anyone getting all up in a fuss about non-anime cosplays at anime cons. Or even ravers at anime cons. Does anyone else get that? Seems a majority of people I now see at anime cons in Toronto are ravers. Is it "we're ravers too"? Or "we're just here for the music"? I'm confused.
Anyway, people should cosplay however they want at whatever con they want. It shouldn't matter. Conventions are for anyone to go to and be accepted. No elitist bs or exclusivity. Just because it has anime in its name, doesn't mean you can't wear whatever the hell you want.