When a game goes up on Xbox live, it has to be updated in some way (that's not counting all the original games on Live). Whether that's online play, scoring, new graphics, new levels, etc, something new must be added to the game to get it on live, they don't just offer up bare-bones roms like Nintendo. I'm not rushing out to download any Live games either (until Castlevania SOTN comes out), but at least on live there's a reason to buy games you might already own since they have new features.
Live also has free demos of games, new content for games, movies, tv shows, etc. Basically it stomps the absolute shit out of nintendo's pathetic online attempt.