I'm of two minds on this sort of thing. On one hand, I do take issue with people claiming to be fans of something without a long history on the subject - I'm a comic book guy. I know a LOT about, say, Spider-Man. I know his backstory, the various writers and artists, his supporting cast, his villains, the real nitty-gritty stuff from 50+ years of comic book history. That took effort. That took a lot of reading, a lot of money, a lot of time, and a lot of dedication. You can like Spider-Man, but calling yourself a Spider-Man fan just because you watched a few movies?
It's like faking your credentials. Doesn't matter if you think, like, biology or history is cool - you can't just hang a diploma on the wall. The people who actually went through years of schooling get to hang up a diploma. They earned that shit.
And a lot of that has to do with social status. If you're a nerd - if you're not popular, you don't have many friends, you don't have a lot in your life going on - your subculture becomes your life in many ways. That's what you've got. So when people who have a lot more going on and live in the mainstream come along and take that away, what else is left? If being "Batman Guy" is a huge part of your self-identity and suddenly everyone loves Batman, you no longer have an identity. That hurts.
But on the other hand - at this point, I don't care. I have a lot more going on in my life than just nerdy shit and reading comic books and playing video games, so I don't really care too much about fandom. And it has some perks, too - being a guy who knows a lot about the new Marvel stuff definitely helps with your popularity nowadays. But since I was that guy at one point, with so much invested in my "nerd cultures," I remember how it feels to have so much invested there.
So it's not really something I feel anymore, but I understand where they're coming from.