Hmm, I dunno, I can't see it being used all that effectively. I mean, if it's done really well and doesn't take away from the main story, it can work, but ultimately it seems like it would be just a tacked on thing.
UnLESS, it's Co-op, I can see Co-op working very well if done right.
I think its always great to add MP to a game. It adds variety to game and a great replay value. So when you finish the game you dont put it away. For me, MP is always good in agame, and in Uncharted 2 i think that it will be a good experience.
I don't think that it needs it, but if it's well done it will be a welcomed addition. I bet it will resemble Gears very much so, which might be nice because I've nevered played Gears and I hear it's fun.
It all depends on how the game turns out. If the single player is anything but exceptional I'm going to blame its shortcomings on wasting time, people, and resources on multiplayer. Uncharted IS adventure. Why do they have to shoehorn in multiplayer in a game that isn't team-centric or lends itself to the style?