People categorize each other based on visual attributes like age, gender, hair and clothing style, and skin color. We also use political, religious, and social ideologies, musical, cinema, and leisure time preferences, and socio-economic class differences to divide ourselves up into different groups. The tendency to categorize and divide has more to do with the basic functioning of the human information processing system than bias or narrow mindedness on people's parts, and it is useful in allowing us to function effectively in the complex, dynamic, and often dangerious environment we call the real world. However, it does have it's drawbacks. It's long been known that people have a marked tendency to identify themselves with a group, identify people who are not members of their group as members of an out-group, and assign negative characteristics to the out-group. Moreover, we tend to spend most of our time with people who are like us. In other words, we hang out with people who look and think like we do and tend to think about people who are not like us in negative terms.
A good deal of this negative thinking about others is due to simple ignorance. Either we never come into contact with certain kinds of people or when we do, we're playing our part in a socially scripted interaction like teacher - student or customer – retail employee. We don't interact meaningfully with many different groups of people and have little idea who they are or what their life is like. Rural Midwestern farm wives don't know very much about born-and-raised East Coast college students and vice-versa. Teenagers know next to nothing about what life is like for their grandparents and grandparents know little about what it's like to be a teenager in the world today.
Negative stereotypes are most effective in shading our thinking when a person has little or no meaningful experience with people in the stereotyped group. It's more difficult to think "I don't care very much about Xs" when you know and like someone who is an X. This is where MMOs have such an insidious and valuable effect. Visual cues are by far the most important sources of information we use when categorizing other people. Age, gender, skin color, socio-economic class, clothing and hair style are all visually salient and are important sources of information when we divide ourselves up. However, when you're playing an MMO, all of this information goes away because you see the avatar, not the player. You have no idea that the Orc shaman or the Kerran monk whose company you've been enjoying for the past hour is a 48 year old housewife from a Minneapolis suburb or a 23 year old clerical worker at an insurance agency in Washington DC.
Will spending time together in an MMO give people deep insight into other people's lives? Unlikely. But it does result in people who would never even consider spending time with each other if they passed on the street enjoying each other's company. People talk to each other who wouldn't otherwise do so. A corporate lawyer is unlikely to spend his lunch hour with the kid standing in front of him in line at the fast food joint who blew off school for skateboarding but the two of them might spend an hour or two having fun together in Azeroth or Norrath. The kid isn't likely to understand how an hour whacking monsters gives the lawyer some relief from the relentless pressure of competition at the law firm and the lawyer may not understand exactly what was meant when the kid said "fing yoinker" after someone ninja'd the resource node they had just cleared of mobs, but they both know they had fun together and that has to be a good thing.
Everyone who has played an MMO for any length of time has had the experience of finding out that someone they enjoy playing with or have befriended in-game is radically different from the kind of people they hang out with in real life. When this happens often enough, even the dullest among us can't help but think "Maybe my ideas about these people have been too limited because it turns out I really like some of these guys." Is this a valuable side-effect of playing MMOs? Indeed.