Playing a videogame requires some level of physical involvement, but watching a videogame being played typically does not. This is one of the more interesting dynamics of playing in front of an audience; for most games, a spectator can transition to a player at the drop of a hat (or controller, for that matter). I am very pleased with modern games offering to integrate your average couch-mate into the game. Windwaker allows a second player to use a Gameboy Advance as a hint-device when plugged into the Gamecube, Guitar Hero and Rock Band offer drop-in/drop-out modes, Super Mario Galaxy has its Co-Star mode. The Nintendo DSi XL was designed with spectating in mind; the increased screen size lets other people watch more easily.
I think this trend is definitely a boon for gaming in general. Games are always more interesting with other people involved, and including small roles for secondary players in a primarily solo experience means that even non-gamers can pick up and play as their interest lasts. I emphasize “small roles” rather heavily; it doesn’t take a lot of effort to give a “spectator” a role without having to re-allocate development resources or revamp an engine. The industry is already beginning to cater to “casual” gamers; but this way they’ll at least be experienced to titles that wouldn’t normally be associated with the “casual gamer” demographic. Online multiplayer is great and everything, but most single player games could benefit from a set of extra hands, and relieve the guilt that most gamers feel when they’re “hogging the TV.”