Earlier quoted context omitted.
One game I particularly remember is Super Mario Galaxy 3D on the Wii. It had a two player mode where the second player didn't control Mario, but instead had the ability so interact with the environment and player in limited ways to help. I wonder why there aren't a lot of other games that do that.
Game design is hard and asymmetrical game design is incredibly hard. Game developers can hardly make symmetrical cooperative systems that properly take advantage of the multiplayer aspect. Borderlands, for example, doesn't add unique systems that take advantage of the fact that there are multiple players. There's no unique mechanics to bosses or enemies. There's not really interesting synergies that you can take adva…
I'm not sure how frequent this is, but when I was little we (me and my brothers) had limited time to play, so we would play our time and watch each others, sometimes completing long missions in Ground Control by playing one after the other. When you look at it this way, having something to do instead of just looking at the screen (which was already plenty of fun) is huge.