Seems a little hit and miss. While I agree with multiple weaker creatures are each weaker than a single stronger foe, he seems to neglect the fact that multiple foes get more combined actions between them. For example, 3 balors (or make that 6, if you have the 3 balors summon in 3 more balors) can easily daze-lock the entire party with blasphemy (and then alternate that with other attacks like implosion or dominate monster), something a single demogorgon would have trouble accomplishing. Likewise, I daresay from experience that 2 wizards are much tougher than a single wizard 2 lvs higher, simply because they get to unload twice as many spells.