I have to say that I think the issue of violence in games isn't really important at all. Or rather, it's about as important as violence in movies, or violent music lyrics, or violence in books, or playing cops and robbers. The point is that if, say, 12 year old kids have trouble distinguishing between make-believe violence and real violence, their parents have bigger things to worry about in their kid's development than violent games.
On the other hand, to take a more extreme example, if a 4 or 5 year old is playing, say, Manhunt, I also don't understand how that might not be exclusively the parents' fault, with or without the existence of government mandated age raitngs.