probably, but if it will be a sort of "fake-battle" like some quests of mop, used only as background when you have to do something else like killing a quest boss (npc vs npc that does not count if they die as they spawn infinitely) i don't see a big problem here, they can use for other classes various fake "followers" or , for example, for DK an undead army or demons for the lock. it would be only a cool visual thing.
Too many NPCs going at it would still cause issues. Players would complain that these "Background" NPC's are cluttering the battlefield for no good reason, and using up their computer resources.