Maxperson
Morkus from Orkus
I never said all of the monsters would be hit. I said more would be hit than if you just had the smaller group of monsters. Do you have 50-100 yard rooms in dungeons? 50!100 yard wide taverns? And do on.The exception that proves the rule. Yes, if the referee is an idiot and drops all the monsters into fireball formation, you happen to hit a lot of monsters with a perfectly placed AoE and you happen to roll well enough to kill off most or all of them with one hit, then and only then will it matter.
The vast majority of encounters happen I'm fairly small areas and making groups of enemies larger means that fewer of them can get out of area spells by spreading out
Unless the DM contrives to have combats happen outside somehow, what I said is not even remotely close to being a "corner case."