If you don't have things like in-game guild systems or other ways for groups to define themselves, then they will have to do so outside the game, and that means that you're not helping them enough. Yes, obliging players to depend on one another and forcing them to interact helps form those bonds, but they do happen without forced grouping or interdependence. So, encouraging group play is always good, since it is a multi-player game. But requiring it may be overkill, if just encouragement gets the job done. A society encompasses many types of people, and the strongest societies are those that embrace that range of difference.
Homogeneous cultures tend to die off.
So conflict and different viewpoints are important. Whether the conflict takes the form of player vs player combat seems immaterial to me, as long as there are mediums for competition of some sort. Beyond that, I feel that the ability of players to engage in player vs player combat encourages group activity and socialization. The bad guys are forever getting together to do raids, the good guys are organizing police forces. It adds a very interesting dynamic.