Good thread. There are few things that people are as irrational about as their choice in bands. If you're cynical of course, you could just say that people listen to music based on their social groups in total conformity 90% of the time, and leave it at that. I'm a little cynical, but I do see the occasional free spirit.
Anyway, I once said I liked Green Day, when I was 15. There was some "hottest 100" radio show we'd gathered to listen to.
My friends basically said: "Green Day's meaningless and they use nothing but power chords". I thought this was a pretty stupid thing to say, 'cause you could tell from the lyrics Green Day were pretty political, and I didn't think it made sense to talk about music being worse just because it's easier to play.
To be fair, I said the "meaningless" bit about their favourite band too. With my band, you could hear the lyrics, and I had yet to make out the Karnivool ones my friends liked until I read them later. If you can't even understand what a band is saying, it's probably a bit fair to say they're meaningless. That, or you need a hearing aid.
I grew to like that band too, and still do. I'm not that into Green Day now, though. 'Twas a phase, and I hadn't listened to much music in those days.