Agamon
Adventurer
I think the real answer is that you largely ignore the rule; just keep it a little bit in the back corner of your mind when you're talking about a lot lot of things. The basic concept is fine - "If a creature has a mechanic that scales better with allies, don't throw too much at the PCs", but the execution "Don't even bother to check if it has such mechanics, or what you're using, just start doubling and quadrupling xp all over the place" leaves a lot to be desired.
This is my conclusion. The DMG should do away with the math regarding this and just explain that being outnumbered makes things tougher.