On the whole "it's only music" is trite and useless, but it can work occasionally, especially when someone acts like you just shoved a broomstick up their mother's arse for incorrectly indetifying the motivation behind a set of death metal lyrics or something...if you feel compelled to write a thesis on the images conjured by a thirty-second piece of a twenty-minute song, more power to you, just don't act like an anus.com about it.
As for Slipknot being locked out of MA, I say accept them even if they're about as metal as a incontinent squirrel, just because the negative reviews would be a great source of laughter.