My brother was one of those. And I hated it. I hated it so much...
I always had to listen to those obscure Metal bands and he also told me to only wear black if I really AM a Metal-fan and that real Metal-fans have black hair and so on and so forth. -.-

But I have to admit that SOMETIMES I also tended to think like that. Like: "What? He/She listens to a band that's become popular? How untrue!"
... Then I started to listen to much worse bands and realized that I was too narrow minded and intolerant.

I've come to ignore those people. And I don't tell anyone what bands I listen to anymore. And I surely won't ask anyone about the name of the band he/she listens to. (Except when the music REALLY is good!) I've grown tired of listening to people talking about music.