Let's give you an example...
There's this area, lava-ey... the developers made 3 paths...
A lets you fight 20 dudes
B lets you stealth invincible dudes
C lets you walk over lava
Probably a lot of developertime went into this, area design, encounter design... it takes up a large chunck of the map... let's see what XP each patch gives.
A gives you... 20x 700XP and the 5000XP quest reward.
B gives you 5000XP. C gives you 5000XP.
Congratiolations, you just voided your work, and instead of 3 options, there really only is one. The only way to force people there would be cheap carrots (say, achievements) but people would still clean A afterwards since it's just too much XP to leave.
A system that would have rewarded "go from A to B, no-matter which 3 paths" would not have that. All 3 options would be equally valuable, and a beatifully crafted area would not be nullified by one of it's path being superior in every way.
And yes, I and many others would still fight... since stealthing the invincible dues is really harder, as you would know if you tried playing said area.
Just imagine how much FUN playing Luculla Mines "avoid the Death Kinght" for a full 50 hours... not so much eh. You probably would do the combat since it's much more fun... and not torment yourself playing just 'cause it gives no XP'... studies probably will show you people will go from A to B with the path of least resistance after all.
Also, look at all the fun fixing Larian had to do in beta;
* Exploit; Respawned enemies gave XP.
* Exploit; Summons gave XP. Full version still even has a few of these.
* Exploit; Killing civvies gave massive XP. Severly curved now, but it's still quite some. And yes, people will murder entire civilisations just for experience...
* Issue; Peace didn't pay off! Charisma XP was invented.
* Resolved issue-new issue; Charisma-claiming and then killing everyone was born.
Etc. etc. etc.
All that time that could have gone to better use if the system was solid from the start, as it appears PoE's will be.
