Agreed. I'm earning XP to make myself stronger, not to level up every foe i fight. It's fine and dandy to have leveled humanoids as officers, elites and commanders, but the bulk of their forces should be in low level grunts. Great Cleave should NEVER be allowed to seem weak, which is exactly what tends to happen as the tribe of 1st level orcs fuse together and becomes a squad of barbarian6/warrior2 orcs.
You bring up an interesting point.
How many people really spend time leveling up monsters from the MM in 3e?
I don't.
I choose to use Orcs against the PCs because their CRs add up to the difficulty I want for the encounter (and presumably befits the campaign). Not because I modified them so their CR adds up to the CR I want for the encounter.
When folks complain about all the time it takes for encounter prep, perhaps they're adding class levels and the like to modify the orcs.
While now and then I might make a leveled NPC out of a monster, that's rare. If I want a CR5, I go to the listing of monsters in the CR5 range and pick something (or roll it).
To me, this is kind of how pre-3e was, except it was Hit Die not CR that was the yard stick.
As to the other point about PC leveling is really leveling up the monsters. Uh yeah. What game isn't? What player REALLY wants to level up so he can go beat up all the monsters he beat up in the beginning of the game, except this time he's higher level. While an easy encounter is a good change of pace, the people I play with wouldn't be happy if everything got easier.
So sandbox where the PCs choose to go to a harder area or whatever, the players I know are going to hit the level appropriate stuff and run away from the too high level stuff.
In which case, its the same effect. Players tend to do stuff around their level.