I think there is a big difference between "let's design lucrative, dangerous areas into low level zones and have dangerous but avoidable roaming threats" and "let's have a buncha deadly level 40 monsters with aggro on the way to some poor level 10 chump's next story quest."
Xenoblade Chronicles is a good example of this. The game is basically built like a single-player MMO in zone structure, has a basically linear zone level curve, but each zone past the tutorial has all kinds of things like side-caves full of level 99 monsters and goodies, giant roaming high level enemies you can avoid, neutral mobs who are way stronger than you, etc, etc. Some of them might be "Big mom" monsters chain aggroed to nearby low level monsters.
These types of enemies and areas aren't pointless challenge - they add variety and flavor to zones that would be otherwise be disposable stepping stones that you need to be X levels tall to enter.
ARR would benefit greatly from zone design that incorporates this - and honestly, it already tries to be there. Unfortunately, all those level 45+ side caves full of beastmen and imperials tend to be both pointless to enter and overwhelmingly trivial for any level 50 player.
