What is class balance?

A MMORPG is more than just one game, it is a collection of several similar but different games: solo and group, PvE and PvP. And I am growing more and more unsatisfied with a system which "balances" classes by making them strong in one of these games and weak in another. Problem is that not all of these sub-games are optional. You can't skip solo PvE in a game like World of Warcraft, because it isn't designed to allow leveling by PvP, and you simply can't find a group all the time to level up. So classes like tanks and healers that are "balanced" by making them must-have in groups and worst possible in solo PvE suffer inevitably. DPS classes might have a more difficult time to get an invite into a group, but not all of them even want to group; or they are quite content to only get an invite to one raid per week, while participation is mandatory for every raid for the tanks and healers.

The makers of Warhammer Online explained a while ago that every race will have a tank, a healer/support class, a melee dps, and a ranged dps class. With 6 races that originally made 24 classes, but 4 of them have been cut and won't be in the release on September 18. Nevertheless the basic philosophy appears to be exactly the same as in World of Warcraft. The tank will deal less damage as the melee dps class, and the healer's damage spell will be less powerful than that of the range dps class. And I wonder how that will be balanced. I have a sneaking suspicion that we won't have 25% tanks, 25% healers, and 50% damage dealers, but that 60% to 80% of players will prefer playing a dps class. After all, WAR is a PvP (sorry, RvR) game, and in PvP tanking is significantly less useful than in lets say raiding. And healing in PvP is nice to get, but ultimately optional for the receiver (because of quick respawns), and a death sentence for the healer. Who wants to be always the first to die?

I think that there should be no such thing as a dps class. Every single class in a MMORPG should have exactly the same damage output, only by different ways and means. And then everybody should have some added functionality: damage mitigation, healing, stealthing, whatever. Every class should be exactly the same speed in leveling up in solo PvE, and bring something different to group combat and PvP. I'm sick and tired of having to gimp myself in solo PvE just because I like to play in groups, and thus often choose group support classes. That isn't class balance at all.