I don't really see how that matters. Unless you are of one hive mind you are going to have different exploration impulses and quest priorities and I don't want to compromise. Also having someone there talking to me, unless they remain in full roleplay character, would harm immersion for me.
As I said, it would work best if the world was persistent and the main storyline could progress separately. That way, people could explore alone or together as they felt like - which would make the impulses a minimal issue.
Part of the social experience, however, is that you have to find a way to work together - and that's something I find extremely easy and pleasant when playing with friends. Which is why it's not a problem that we have different impulses. That's the kind of thing I find easy to overcome with good friends. So easy, in fact, that it's never a real issue.
Immersion is a good point, but there's really nothing to prevent you from playing the game alone. It's not a requirement, but a feature for people who enjoy the social experience.
To me, having experiences in singleplayer games often feel too far removed from what happens in the actual world. As such, I find it ever harder to immerse myself in a world that I know is never going to be shared with anyone.
Since I discovered cooperative multiplayer, back with games like Diablo - I've never been able to fully enjoy singleplayer games in the same way as before. The same is true for several of my friends - and we still get together when time allows.
It was really quite profound for me, and I think it's unfortunate that so few people seem to enjoy the experience. I wonder if they've ever really tried playing a cooperative CRPG - but I guess most people here know what a good PnP session is like - and it's much the same kind of experience, except you don't depend on a dungeon master having a good day.
In a linear game within a genre that has less to do with immersion, like Left 4 Dead or Boderlands, I can enjoy coop. Not in an RPG like this though, not at all.
As I said, to each his own.
Though I think you might be surprised if you actually tried it. Have you played a non-action RPG with friends?
Sometimes, having a good social experience can - in itself - be an inspiration for the concept as a whole.