Disadvantage: Gamers/Viewers above 18 have to go through some trouble to get what they want - and this bothers a lot of adults. Still, I believe, for the sake of young people this age rating should be kept as it is.
don't forget that it results in fewer games being sold, too. this further results in developers often crippling the core of their games to make them palatable to censors in order to increase their market share (it's usually the publishers that put the pressure on the devs). Same goes for just about any entertainment medium.
