Thanks guys.

It is really sad to know that these allies causes this (I refer to non-player), once it does not receive any XP from battles.

Some content designs finds a way around this issue by awarding no XP based on Challenge Rating for combat encounters and leave the results merely in a critical path that needs to be breached in any manner possible (solo or party) but successes serving as milestones. They award XP for achievements as flat values so it doesn't matter how large the party is, the reward is the same. I.e. a survived creature encounter would always reward the same XP. And there are both party rewards and class action rewards spread around. TBH, this strategy reflects PnP more closely than the strict CR method that vanilla NWN has propagated and can balance class advantages better since no single class can gain achievement rewards from all possible opportunities (like unlocking a door, dispelling an enchantment or intimidating a government official, as just three examples of class-specific rewards when successful).

Of course, it much harder to design environments around PnP dynamics so they are rare. But they're out there if you search.

At least it's not like games like Gradius and Zero Wing, where when you die, you have to start at the equivalent of level 1. Good luck finishing the game after *that* happens.

Or Castlevania. Die to Frankenstein and Igor (a rather difficult boss if you don't have Holy Water), and you lose holy water (if you have it) and there's no way to get it back without game overing and starting at the beginning of the block. (That game has another annoyance; a random axe drop can deprive you of your holy water, causing the battle to be much harder because the RNG decided it hates you. Also, there's no save feature in the US version, and the Japanese version's save feature would not help in this instance, as I believe save/load is no better than continuing after game over.)

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.