The moment a glowing triangle appeared in the corner of the screen in Uncharted 2: Among Thieves, prompting me to engage in a co-op move, a mild seizure overtook my synapses. I started having flashbacks to Resident Evil 5, the only video game in recent memory I can honestly say I was forced to set down the controller in frustration and said "no, I don't want to continue, because I've had enough." Uncharted 2 does not make the same mistakes as Resident Evil 5.
Naughty Dog's approach was the right one, a proper balance act. It wouldn't be surprising to hear Naughty Dog reveal there were plans to introduce co-op into Uncharted 2's story mode, only to remove the option once they realized how much it complicates and potentially compromises the single-player experience if it's not specifically cared for every step of the way.
There's an example of this approach from a series gamers have demanded co-op from for years: Call of Duty. Infinity Ward will not include co-op in the campaign mode in Modern Warfare 2, despite acknowledging it's a popular fan request. The studio recently revealed that it attempted to implement the idea, only to discover unintended consequences for the single-player experience. Infinity Ward noted the issues but did not ditch the promise of co-op, instead developing a separate mode for it.
“We did it early on; that’s what Spec Ops [Modern Warfare 2's co-op mode] started out as -- as co-op through story,” said Infinity Ward community manager Robert Bowling in a recent interview. "But it really broke the cinematic experience, took the immersion out of it, out of the story and the pacing and everything we’d spent so long crafting. It just ruined the experience we were aiming for, so we took it out, kept the single-player for that one player intact and polished it.”
A-ha, there it is. Separate. There are absolutely instances where dropping in multiple characters into the game world and calling that co-op works effectively (see: Halo), but what we're seeing more and more are developers whose bread and butter are rollercoaster single-player blockbuster experiences finding it difficult to just shove co-op through. Resident Evil 5 had a great many issues that would have made its co-op a more manageable experience, but by and large, it seems Capcom didn't see (or notice) the same roadblocks Infinity Ward (and perhaps Naughty Dog) encountered.
Many times throughout Uncharted 2's story, there were set pieces that made me think "man, wouldn't this be great with another person by my side?" Lo and behold, the co-op scenarios included in the game's separate co-op multiplayer are plucked straight from the game. Rather than struggling to please everybody with the single-player experience, Naughty Dog rightfully took the moments where co-op would have made sense, left that to the A.I. and dropped those experiences elsewhere.
There's no reason someone can't come up with a way to seamlessly integrate both together, maintaining the excitement possible when the focus of the game's design is solely on one person's attention, but there's ongoing evidence that's not the best solution.