I have proposed this concept three times if I'm not mistaken.
It's important because it is a feature for the games, may be the basis of a purchase decision and it would be good to be able to list games that have local co-op capabilities. With this I mean they can be played with one single system, for example Gears of War.
Every time I want to check if a game features it or check which are the latest games that I can play with my sister I go to http://www.co-optimus.com/
However it's good to be able to search for games that have gorillas, I mean, who cares about the multiplayer modes when there are gorillas in the game?