Developers usually explain Nerfs as part of an iterative process of rebalancing character abilities - particularly from a PVP point of view. Joshua Fairfield from Terra Nova has just written a very insightful piece where he points out that it is not about balance at all. The ongoing process of nerfing some abilities and buffing (improving) others is actually about disrupting the equilibrium - knocking players out of certain established patterns of behaviour which are guaranteed to get results. He is spot on. In Guild Wars there is a name for the current popular style of PVP play. It is called "The Meta Game" and the developers constantly adjust skills simply to upset the current meta game. Skills that become extremely popular in pvp play are prime candidates for nerfing forcing players to investigate new combinations. I think that this description of nerfing as a means of disrupting the cosy equilibrium can also be applied to nerfs in the PVE domain - it forces players to abandon established proven patterns of behaviour and look for new approaches.
I can understand that motivation but yet I am not fully content. Understanding the reason does nothing to alleviate the abject feeling of despair when a build you have spent many hours perfecting is rendered useless by the blunt instrument of a nerf.
Part of me wonders why they bother at all. If developers never adjusted skills and abilities what would happen? I guess the worry is that everyone would simply fall into one groove playing the perceived "best combination" and the game would lose vitality and interest. I could live with that. If a game began to bore me I could always move on and play something else. Needless to say that solution isn't as appealing to game developers.
I do know one thing. I wish that ongoing re-adjustments would focus more on "buffs" than on nerfs. For one thing it is less demoralising when your opponent gets stronger than when you get weaker. For another I think that buffing could be more selectively applied. Imagine for example that a certain skill is proving unbeatable when used against assassins. Surely it would be better to assassins a special counter skill rather than reduce the effectiveness of that skill across the board.