Minecraft on 360 is never going to be more than a niche, given the type of game it is. If it's available through Steam, however, the majority of PC players will end up buying through Steam and that population will pressure the non-Steam PC players to move to Steam.
There's precedent for games purchased outside of Steam being available in steam, so there's no technical barrier to giving all existing Minecraft owners a copy of the Steam version.
But when Notch says that he is avoiding Steam due to the restrictions it will place on what he can do with Minecraft in the future, I'm not sure why this would be a positive outcome from Notch's perspective.
