The only thing is that there aren't a whole lot of "multiplayer" games. Some are with Cpu's, some integrate a second player, but I've only seen two that you can play live with people you don't know. Also, that would be alot of work recategorizing the games.
@All, I spoke to the webdev team this morning, and there was a resounding "absolutely, love the idea" response from the team. We'll work on a way to flag multiplayer games, games where you can submit high scores, etc. for the AG3 site.