I'm curious about this. Last night I went into a public dungeon and people were running about killing baddies and generally having a good time, but everyone kind of soloing, doing their own thing. I enjoy the camaraderie of grouping, even if it is short-lived. It gives you an extra element of participation in the game when you can thank a player for helping out, or even have a brief chat. I put out a /say - anyone interested in group? One player replied, not really, which is fine - everyone has a right to play in the manner they choose. But the question popped up in my head: Why not group if more experience can be gained? You can still do your own thing. There is nothing that compels you to interact with the person you group with. You can leave group anytime you wish. If there is, indeed, a significant gain in experience, players can only benefit from grouping.
Personally, I'd like to see more incentives to group that are evident as you're playing. Again, I stress the point that every player has the right to choose to group or not group. Maybe the system is fine as it is, but I do believe more players would realize the benefits of social collaboration within game if the benefits were more evident.