I don't see this being good for fighting games. You need almost 0 lag for those to work, and I get the impression that this system won't be able to support that. And as you said, most PC games have multiplayer already. There's very few that have JUST local co-op, and the ones that do likely require it due to lag/delay issues. Honestly I see this more being used as an "assistant," like if you're having issues beating a boss in a game but your friend has beaten it so you invite him to take the wheel so to speak. In any case it's a cool technology. However, be careful who you give control to.
Let us not forget about this story..