I've often scratched my head at the design decision of "If you're not good enough to beat this, were going to make it harder by removing XP." I just don't understand it.
That said, the game scales pretty good to your current level. I played with a henchie and a constant summons for my main OC play"through" ( quit at the end of chptr 2 bc I love love love multiplayer NWN) and never felt the game was very hard.
That's not the reasoning. It's because the creature AI is so easy to exploit that innovative designers set an XP based on achievements rather than AI and script exploitation. If you've never played AD&D PnP, then yes, it is more difficult to understand how it works.
The OC plague campaign is essentially a series of modules created to orient new players to how the game engine works, i.e. a learning campaign. It was never rebalanced for all the prestige classes, improved feats and spells so since about 2003 (the year SoU was released) the OC was never again able to be played with the challenge Bioware intended and even for that one year grace period it wasn't all that hard.