Having had a go at Saint's Row 3 co-op with a friend yesterday, I must admit it is enjoyable and in some circumstances can work. It's not a perfect system by any stretch...co-op is basically the host's single player campaign with a friend in there. The guest can progress themselves in some ways, but not in others.
The most enjoyable part of the evening for me I had nothing to do with. I went AFK briefly to get a drink and while I was gone, my friend stole and brilliantly re-arranged several cars out of my field of view. On returning, I saw the results and was in hysterics at what he'd done. This works in SR3 because it's a playground for having fun in. Skyrim and TES titles are not. There are consequences to actions and a world that is shaped by our individual decisions.
On the flip side, someone else mentioned Dragon Age and having thought about it, I think maybe it would work there. Why? The more structured, linear approach means players can proceed apace and further, it is a party based game whose gameplay directly works around balanced, complementary team members. Conversations and decisions would be interesting to handle...maybe something like SWTOR, or the host decides...
For those after multiplayer RPG experiences, I've looked around and there are quite a few already out there:
Borderlands, Diablo 2/3, Baldur's Gate II, Neverwinter Nights 1/2, Dungeon Siege III, Dead Island, Sacred 1/2, LotR: War of the North, D&D: Daggerdale, Too Human, Port Royale 3 (also has Pirates!!), Titan Quest, Two Worlds II, Fable III, Hellgate: London and forthcoming titles such as Neverwinter and Torchlight II. Not to mention the entire catalogue of F2P MMOs. Also, cross-genre titles like RE5, SC: Conviction, Hunted: Demon's Forge.