In an ideal game (pure fantasy I know) with such a system though it wouldn't matter when you got there. The encounter would be difficult when you're there doing it and would become static after you've met area-specific criteria. Now having an area of "useless mooks" does serve a purpose. It sits there as a marker of what you've accomplished, how far you've come. Now I would agree that leaving the area there solely for the players that like to one-shot weak things is flimsy, but what about players just passing through to get to another area? Consider it for areas that realistically *should* respawn over time anyways, like encounters with wildlife. There isn't a reason to respawn special encounters, but certain omnipresent things like wildlife should.
Except if you get there at level 1 and clear it out, it forevermore spawns hostile squirrels. If you get there at level 50 and clear it out, it forevermore spawns elite dire bear raveners. That could be a significant difference at some point.
Leaving a weak-ass area as a transitional area doesn't make a lot of sense, because then it's just filler. It doesn't provide a challenge, it presumably doesn't provide meaningful loot or experience, it's just a place you have to trek across and occasionally be accosted by useless squirrels on. If there was any point to letting wildlife attack you on your way, there was probably a point to making sure the wildlife was an encounter and not a nuisance.
The only exception to this is when they're functioning more like resources than encounters, ie you need deer to respawn so you can harvest deer hides so you can upgrade your stuff. That's slightly different, but even then the same basic issues (pointless later on if deer hides are only for low level stuff, for instance) apply.
The example I gave is of a complete group composition, but consider applying the concept to which individual encounters are spawned as well. Lets say at lvl 1 only butterflies and deer spawn. At lvl 5 wolves get added to the list of possible things, but you still get plenty of deer and butterflies. At lvl 10 bears get added to the mix, but leaving the now weaker things still able to spawn.
I was considering that as well; hence the "portions are still useless" bit. Sure, the region
might spawn level 10 bears that actually mean something to you, but they could just as easily spawn a flock of butterflies that serves no purpose whatsoever except taking up room that could have been bears.
I do get the feeling that you're saying weak encounters serve no purpose. I disagree with that specifically. I would say that while you can judge your character progression based on the progression of current encounters, how far you've come tends to be much clearer when you're faced with what used to be a challenging enemy whom you can now mop the floor with. The problem comes when this happens too often or too little. It should be just enough to remind you of where you were then and where you are now, but not enough to bore you.
I maintain that this is far, far too flimsy an excuse to justify basing sweeping game design principles on, though. Again, I can see having
access to lower level enemies in some fashion, but I don't think the occasional satisfaction of "Wheee, these things used to be hard and now they die in one hit!" is worth the irritation of constant "Shit, how many of these are actual enemies and how many are scenery?" and "Okay, gotta hit the group with one fireball to clear out the trash, and then the fight can begin!" and "Why are there so many useless mooks I don't care about in here?"