Wulf Ratbane said:
Switching gears to Eric's post, there are plenty of creatures in the middle-CR range (say, CR7-CR12) that pose a meaningful challenge when presented in larger numbers-- elementals, magical beasts, giants, and undead. You don't need to go hunting around for CR18 monsters when eight CR12 creatures also works.
Part of the problem is that the CR system falls apart once you require too many monsters. In other words, I am much more suspcious about the "real CR" of an encounter with 8 CR 12 creatures than I am for 1 CR 18 monster. Adding more combatants to an encounter tends to slow down play, make the foes more vulnerable to area of effect attacks, and lower the "gear" below interesting thresholds.
Compare an 8-armed orc with 6 HD to 8 1-armed orcs with 1 HD apiece. Both are CR 6 encounters.
Both have the same number of attacks, and most DMs run combats with all the 1 HD orcs having the same initiative. However, many combat conditions and actions (readied attacks, slow spells, etc.) cause the 8 1 HD orcs to diverge in initiative order, leading to a slower combat. The best solution is to make the 6 HD orc have 1-arm with a very powerful blow, in order to speed up play.
A well-placed 5d6 fireball might kill the 8 orcs but is unlikely to kill the 6 HD orc.
The 1 HD orcs are going to have "boring" gear like masterwork weapons and armor. The 6 HD orc might actually have a unique magic item.
The 1 HD orcs require a larger battlefield, making for less logical dungeon design if repeated for the majority of encounters.
These are just some examples and I'm sure there are counterexamples. My general point is that I wouldn't want to have more than a couple of encounters at each level be with 6+ combatants. The sweet spot for me is 2-3 opponents in most encounters, and I think this is reflected in the preferences of Dungeon's editors as well.
Given that, the problem of most higher level threats being extraplanar or dragons remains.
--Eric