Decided to put a little explanation in as to why I enjoy co-op stuff in a fantasy setting.
I started playing Age of Conan, an MMO, when it released. I really enjoyed it, and had loads of fun but I was mostly doing quests on my own. Occasionally grouping up to do other things. Now in AoC you level up pretty damn quickly (compared to other MMOs) and once you hit the max level (80) there was literally nothing to do other than engage other players in some PvP or do one of the two raid bosses. So, what does one who doesn't like raiding or endgame pvp do (relies too much on gear over skill, hence my dislike). I role played, made friends and eventually decided, with a friend, to create a pair of new characters.
So my friend, called "Sally" lets say, and I create new characters. We had a very, very rough idea of their background. Just a general 'they have this sort of personality' thing. Then we started playing and always did quests together, exploring the world and role playing through out it all. Obviously we had a chat box so we could type out emotes and such, which naturally helped the RP. Anyhow, so we go off and level up to 80 over time, having had plenty of RP and PvP fights along the way which we took as in-character things. It was -loads- of fun. Now we're at endgame again though and its as boring as before, only I know have more RP which is awesome. So we make another pair of characters who were completely different and play through the game again.
Despite all the quests, locations, enemies and so on being the same, our characters weren't and how they interacted with one another was vastly different than our previous characters. So, simply put, it made the game enjoyable beyond its 'normal' lifespan for me and Sally.
So lets take Skyrim. I'll play through it as a warrior, finish all the factions and main quest, do all the side quests I can find and in general just explore most places. What to do then, when there is nothing left to do? I'll start again of course. Maybe as a mage, or an archer or what have you. The quests will still be the same more or less though, maybe a different NPC will give them, and the location will be different - yet regardless, I'll have explored that other location previous before too. So the game won't be as enjoyable the second time, less so the third time and so on.
Add in co-op in Skyrim, where bother players (with one being Dovahkiin and the other not) create their characters, designate which player gets to be Dovahkiin, and off into the world you both go to role play and explore. Even though I'd know the quests, the enemies, the locations and everything else. It would be a refreshing, fun experience solely because I'm now doing it with a friend who, gameplay mechanic wise, would be considered as an 'NPC companion' just played by another person. Beat all the quests or clear out all the dungeons and get bored? Have fun terrorizing a town or something, or make new characters who again are different.
Its like reading a novel in a way. You read it the first time and learn everything. You read it the second time and nothing surprises you at all anymore. Swap out one of the companions in the novel with another, totally different character, and it becomes interesting to see what would be different. Obviously this only really applies to those who role play their way through the game, as actual NPCs and plot elements wouldn't change at all of course. Just let the players beef up enemy difficulty if they wish, so to succeed you have to work together, use tactics, etc. Becomes a new way to challenge yourself in the game.
My two septim. ^_^