ccs
41st lv DM
The game itself doesn't care about class/character imbalance. The designers do, somewhat. Many players at individual tables care.
The imbalance I've seen has less to do with inflicting damage in combat than it does with overcoming challenges in the other pillars of gameplay.
Let's take a hypothetical situation:
The DM picks up a adventure for his mid-to-high level group (say 7-9th level). The adventure requires the PC to discover the whereabouts of the BBEG, travel there under time constraint, launch an assault on his underwater base of operations, and has a final battle inside an iceberg that inflicts continuous cold damage on anyone unprepared. The adventure is designed to exploit the abilities of a well-rounded group of 4-6 PCs with at least one martial combat machine, one arcane caster, one divine caster, and one skill-based PC.
Scenario 1: the group is well-rounded. The adventure plays fine. Divine and arcane casters handle the discovery, travel, and life support needs of the group.
Scenario 2: the group is composed only of divine casters. Discovery, travel, assault are fine. Traps and alarms are triggered, but survivable. The final battle is a little tricky with the reduction in combat performance.
Scenario 3: the group is composed only of arcane casters. Discovery is a bit harder. Travel and assault are fine. Secondary tactics like summoning prove very useful during the assault to limit damage. The final assault fails because the environmental damage takes its toll. The PCs manage to escape with the last of their spells rather than TPK.
Scenario 4: the group is composed of martial combatants and skill-based classes only. Discovery is a challenge that ends up burning a lot of time. Travel duration exceeds the available time so the mission fails. That's OK though because even if the group managed to reach the underwater base, they can't survive the trip to the entrance and the environmental damage inside the iceberg would likely cause a TPK since escape options for the group are limited. The actual assault and final fight would have been a breeze though, should the PCs be handed a route to it!
Of course it helps you make your point that those hypothetical players in #4 are apparently just stupid....