Imagine a tiny change to the code of World of Warcraft: When in a group, players would get twice as many xp per kill as they get now. That tiny addition of "x2" to the code would completely change the face of WoW. It would suddenly be far more advantageous to level up while grouping, instead of soloing. The forums would explode with some people calling this "forced grouping", some people would continue to solo, but a large number of players would simply adapt to the new situation and group a lot more on the way to the level cap. The dungeon finder would get used more, because, hey, if you already group, you might as well tackle the content with the good reward. But even when just questing and killing 10 foozles, you'd throw an invite to the other guy you come across killing the same foozles.
Chris Smith from Levelcapped this week responded to Syp's question of "If we are lazy and resistant to being social in MMOs (the path of least effort, etc.), is it the game’s/devs’ responsibility to encourage — or even force — us to do so?", by wishing for an Unsocial MMO. That resulted in an intense debate with Spinks on Google Buzz, who wants developers to encourage people to make friends.
Although some people tend to respond to this sort of question with polemic, like "if you don't want to play with others, play a single-player game", the problem is actually deeper than many people imagine. There is a sharp disconnect between short-term and long-term motivation to play MMORPGs. While in the short term players tend to react strongly to rewards, many surveys have proven that social connections are one of the major reasons for long-term motivation. The question of "should a game encourage/force people to be social" thus isn't just a philosophical question, but also fundamental to the perenity of a game.
After 6 years of World of Warcraft, the activities that occupy the player's minds are mostly social. There is far more discussion about raids and pickup groups than about solo gameplay. Solo gameplay is considered "not important", and dismissed as the tedious obstacle you have to play through with new characters before you can get to "the real game", which is social.
But while you might keep playing because of your friends, and your best memories of the game are about moments shared with other people, your worst memories probably also involve human interaction. Obviously the biggest idiots and jerks you meet in the game make for the best stories afterwards, thus what one reads isn't necessarily representative of the average pickup group. But grouping with complete strangers admittedly has its pitfalls; and a combination of the "barrier to entry" of forming a group in the first place, and the risk of that group failing to reach its objectives, makes grouping often less attractive than soloing, especially for short play sessions.
Using rewards in a clever way can overcome these obstacles. As I repeatedly wrote on this blog, World of Warcraft is failing to do so for groups outside dungeons, which explains why leveling has been nearly synonymous with soloing before the dungeon finder lowered the barrier to entry into high-reward group content during leveling. Many quests get actually *harder* to do when you group up for them, and the xp per hour in a group is lower than soloing the same content. Furthermore most leveling content is easy enough to be soloed. While changing the rewards could turn leveling in WoW into a more social affair, some players resent such social engineering. The term "forced grouping" is often applied for situations where in fact nobody is forced to group, but grouping is just the more efficient way to gain rewards.
Blizzard is currently introducing a system of rewards that encourages players to guild, by handing out guild rewards. It isn't quite clear yet whether that will work in a positive way, we will have to watch the system in action. There is a certain fear that the system inadvertedly favors huge guilds over smaller ones, which might actually end up counterproductively destroying social interaction, and lead people into huge, impersonal guilds instead. On the other hand the reputation you gain for your guild should prevent some of the worst excesses of guild hopping.
In summary I think it is a good idea for developers to use rewards for social engineering players into groups and guilds, because this can improve the long-term stability of the game. But as social engineering is difficult, care has to be taken not to inadvertedly do more harm than good. It is okay to offer better rewards for players to play together, but not to a point where solo players would feel punished, or where impersonal mega-guilds have huge advantages over closer knit small guilds.