Why wouldn't it work for Uncharted?
I think it would be cool if you could play the secondary character to help solve the puzzles. Say have the second person trip switches which would make some puzzles easier to get through. But to balance the game, maybe ramp up the number of AI enemies and make it harder to get through the combat than hitting and running with just one player.
Hopefully, LBP will influence other games to add online co-op components where additional players make getting through levels easier but with more difficult AI to balance the game.
Heavenly Sword could have Nariko fighting a bunch of enemies with Kai picking off stragglers here and there. But Nariko would also have to protect Kai as well since she can't do hand to hand combat.
Now would all of this make these singleplayer games sell better? Not necessarily but it would offer some replay value which minght attract some buyers who otherwise would have only rented or played and sold these single-player games, which seems a typical pattern.
On R&C, yeah the online modes on the PS2 games were derivative of shooters. I think a co-op mode might have been something different because there are some singleplayer levels which were hard to get through.
Then again, maybe the designers don't think it's in their interest to make it easier for people to complete the games with the aid of a co-op mode. I think the fun factor would be higher though.