Okay, so Commoners are useful? Having a PC play an awakened rat with no class levels is useful? Because those both satisfy the inequality in question.
Clearly they are? Its straight in the word. Use-ful. Having a use. And they both do that. Perhaps not as much as a lot of other classes. But then you need to change your argumentation to less or more useful. Instead of the binary useful or not useful.

... and do you really want to argue that the NPC fighter should be the model to aim for? Then try to argue with the NPC Wizard and the preselected spells and equipment as a counterpoint. Thatīll get funny fast.
Yeah i already made the point that none of the standard NPC classes can handle a Balor on their own. Or at least are highly unlikely to be able to do so. PW-Stun and the save-or-die abilities will handle the casters, and it can chop everyone else up easily.