One thing that BOGGLES my mind is how inconsistent it is [the general concept of 'age checks/parental advisory']. Granted, it's voluntary, but still.
One of my favorite bands, GWAR, is very much 'parental advisory.' You can tell just from the song titles. Not an issue for me, since i'm of age, but how about this: in most venues, the live show is 'All Ages.' I'm going to see them next week, and technically I could bring my 18 month old son for free [kids under 2 don't need tix to concerts.] Not that I would, of course, but last time I saw them, I saw a number of 12-14 year olds. I'm not saying age necessarily implies maturity, but how can it make sense to card someone for wanting to buy a CD or a game, but not care for them to see it live?
And yes, I think the 'why it's rated this' would be good for music, I do appreciate that feature on the video games.