IMO some games don't need co-op/multiplayer of any kind, way I see it, Uncharted fits into that category (that and any RPG)
True, they don't need to have it. But, when done correctly it can make an already awesome game even better, like Secret of Mana. That being said, Uncharted is still one of my favourite shooting games recently.
@Squilliam - Given your criteria for purchasing a game, I'd recommend giving PixelJunk Mosters a try. Although, I'm unsure if there is a demo available.