I've always figured that NPC classes were there as a sort of generic filling for the character pastry.

For instance, if you are storming the castle of an epic level villain, you could probably justify him having guards posted around that are around 20th level. But generating dozens of Fighters, and picking feats for each of them is tedious.

Far better to give them say 5 levels of Fighter for a bit of customization (some can be archers, other might be pikemen etc), and then filling in the difference with levels of Warrior. They'll still get a good attack bonus and fort saves, but will be easier to generate and run, and their somewhat lower hit points mean that the players can plow through them like the mook army they are.

Experts and Aristocrats have their uses as well. Can't see the point of epic level Adepts and Commoners.