The only qualm I have with increased popularity is the new fanbase that tends to get drawn in. I understand that each new era of a band draws in new fans. However, the more mainstream attention the band gets, the more chance a 'mainstream-minded' person will get into them.
You tend to find that the more obscure a band is, the more effort the fan has to look into order to find them. That implies #1 the fan really is a lover of music, or connoiseur or whatever wanker word you preffer and #2 they show incentive and that's indicative of an 'enlightened' or perhaps 'intelligent' person.
The more a band gets into the mainstream spotlight, the easier it is for people who don't really give two hoots about music to hear them. They may like the chorus to Isolation Years or something because it's really sombre and catchy, but their experience of music would only be limited to that narrow appreciation.
I'm not really sure if I'm being clear at all, but basically what I'm saying is that I dread the day that I see people practicing their karate moves (xcore) in the pit of an Opeth gig *shudder*.