Actually the point of 3 is not to progress in a predictable rate. Take Gothic 1 and 2 for example. There were areas around your level that you would follow and harder areas that you would visit later when better equiped and of a higher level, powerful enough to rise to the challenge. However you could go to those difficult areas from the start and fight battles that would be much more difficult if you decided to take them on later on the game. That works in 2 ways. First it's a test of skill (and since Original Sin is turn based and not real time a test of strategy and tactics). You get the satisfaction of slaying that troll at the first chapter although you were supposed to do it in the second. The second way is that stronger opponents usually mean better rewards, xp and item-wise so if you managed to accomplice the feat of going off the beaten path and surviving that would mean your character would grow more powerful and make future challenges easier.
However if that is coupled with diminishing experience it creates two different problems for the player. First, lower level areas will be both easy and not rewarding at all making it a chore to go through them. (Kingdoms of Amalur had that problem in a huge degree). Second the player feels punished for straying from the main path instead of feeling rewarded for a difficult accomplishement.
It's also worthy to note that taking on difficult battles many times means that you'll burn through your consumables so it's not like they don't have any drawbacks. However that is a choice the player makes. Is it worth to spend my 5 mana potions to kill that fire demons and then have none for the easier but still not trivial encounter? That adds options. Forcing the player to follow a certain path if he wants to maximize his power is limiting them.
Closing a great recent example was the quarry filled with Deathclaws in Fallout: New Vegas. You could visit it very early in the game and you would normally get slaughtered. However with enough ingenuity you could mabe kill one or two even at low leves, using up your explosives or sniper ammo or some other resource.
