There is already a great deal written about game balance, such as how to achieve it, its importance, and even the pitfalls in dealing with it. Game balance is one of the most difficult areas of game design. Both paper & pencil and on-line multiplayer games need to ensure that each class or profession is balanced. This is vital to the overall health of a game and while it may be an impossible goal, it is a goal the designer must strive for nonetheless.
Game balance imperfections do not often show up during theoretical discussions or even during internal testing. Games are often pushed into a live environment with flaws that make a class out of balance with the others. With the dynamic nature of MMORPGs, changes can be made and pushed live as a patch. When such a change occurs and a given class is reduced in ability to bring it in line to other classes, the term used is "nerf".
While the hard-core role-players might pick their class based on a preconceived idea and not on the over/under powered nature of a given class, the out of balance nature of a class cannot be ignored. When these nerfs appear, players affected cry and whine in frustration demanding to understand why the change occurred, and more importantly, why even bother with game balance. A select few even try to argue that their out of balance character really wasn't and instead of reducing the ability of the out of balance class, the other classes should be raised to compensate. So a class was more powerful than one or more classes (or all of them), what is the harm in that?
At its most fundamental level, when one class possesses the ability to outstrip other classes in terms of advancement potential players will flock to that class. The other classes cease to serve a purpose. To look at it another way, you have 2 very distinct and different classes, each with a different set of abilities. A third class can do both just as effectively, why would anybody run the other two. In order to balance the "draw" of the third class, its ability to mimic the other 2 classes need to be reduced. Yes, this third class can do both, but at the expense of the other specialized professions. Raising the ability of the two specialized classes can set a bad precedent. It encourages even more whining by the player community and serves to increase the overall power level of the game, requiring even more powerful opponents, something already a problem in long term games as an aspect of mudflation.
When a game possesses a player versus player combat option, the need for class balance is even more pronounced. There has to a check and balance. Each class should have something it can defeat and something it cannot – someother class must do that.