With the announcement of Bioware's Star Wars: Old Republic MMO it seems that the prevailing trend among triple A MMOs is towards a much more singular experience, one in which a player may spend hours in a persistent, online world without so much as talking to another human being. Warhammer Online, whose developers and critics alike, touted the public quest system as a way of making grouping dynamic and intuitive has ironically led to situations where those in groups don't actually interact with each other. Having abstracted away any need to organize a group among other people, the "group" becomes little more than the people who happen to be in a certain geographic area at any given time.
Another future MMO thinking along Bioware's lines is the Star Trek Online from Cryptic studios. Here instead of a companion character, the player has his own companion bridge of friendly NPCs. While not controllable in the same sense that Bioware has hinted its companion characters will be, these NPCs will accompany the player during both space and land exploration
A current example worth considering is Spore and particularly its new space addon pack. Spore was presented as a massively single player experience and in that regard it excels in populating the game with user generated content. The space addon goes one step further, allowing players to create their own quests for others in the space stage of the game, which are then disseminated to the worldwide community of players. You'll never directly interact with another player in Spore but you will benefit from the collective actions of millions of others; and these efforts will in time create more dynamic, interesting content than the largest team of MMO developers could ever hope to achieve.
This idea of more accesible single player experience within a massive world isn't new - Guild War's henchmen system pionered it in the MMO space - but the focus upon it as a game feature, a selling point of the experience, is. Earlier efforts have been presented as sort of apologies for the drawbacks and limitations of group play. The focus on companion characters as an integral part of the player experience changes the social dyamics of gameplay. Presumably your character's weakness demands that you interact with other people, become invested in their stories, in order to advance your own. What happens when its just me, myself, and irobot?