"WHY is Blizzard so concerned about balancing everything? Either they're oversensitive to people's complaints - which I doubt - or they're roleplaying druids interested in maintaining the "balance" in all things. :)"

Maybe oversensitive, but maybe they're just sensitive enough. I didn't PvP too much or too seriously pre-BC, but people were always, always arguing about whether paladins or shamans were overpowered. After the nth time a paladin bubble-hearthed just before you could finish him off, or a shaman frost-shocked and earthgrabbed you for five minutes straight, a lot of people would just shrug and try to avoid those classes or reroll and play as what they saw as OP (and probably learn that the other side has problems too), but maybe some people were quitting in frustration. I don't know, ask Blizzard.

It actually does make the game less fun, and therefore albeit indirectly less profitable to Blizzard, if some things are imbalanced. Servers with too high populations have lag; servers with tiny populations have wasted capacity. Servers with severe faction imbalance have one side always lose at PvP and zero-sum world events, which isn't fun for the smaller population. If a class gets OP, then people who play other classes will be jealous. If a class gets nerfed to the ground, fewer people will play that class and those that do will be less happy. Blizzard doesn't have to be role-playing to notice any of these problems, obviously.