Damage is only part of it. Group buffs, targetted buffs, special abilities, and spells are all included in balance along with damage and tanking ability. For instance, Guardians have a number of good group buffs that we lack, like their +defense and +hp group buffs. Monks have one group buff, shared with all other fighters, and the +avoidance buff, shared with all other fighters. That suggests that we should be better than Guardians in a different area.
Weighing all of these factors against one another is where balance becomes difficult, and it is very complex when you consider all of the different scenarios in the game (solo, small group, low DPS xp group, high DPS xp group, single mob epic raid, epic raid with adds, fleeing from gnomes with the plague, etc).