Still, the important thing was that the mechanical differences represented actual, substantive differences between individuals. Orcs had shamans and witch-doctors instead of clerics and mages, and the difference in their stats and abilities was because they were different things (rather than reflecting their narrative importance).
That's not exactly right.
For example, in AD&D, you can have 3 human beings; a hireling, a henchmen, and a PC. For fun, let's give them identical stats. The PC and the henchmen have class levels, the hireling doesn't.
They all go on an adventure and survive. The PC gets a full share of XP and can increase in level.
The henchmen gets less than a full share of XP, but still can increase in level. PCs can only have a limited number of henchman, as determined by their CHA.
The hireling doesn't get any XP, and can't increase in level. PCs can have as many hirelings as they can pay for, but they don't improve.
The DM & player at some point, might change the hireling into a henchman. Note this has nothing to do with any inherent quality the henchman has. This change is predicated on a) the player desiring it and b) the character having enough henchman slots free.
So the substantive difference between these three don't reflect anything in the game world. The difference reflect their respective roles in the 'story', i.e. star, sidekick, extra/redshirt.
(I know you said your experience was with AD&D 2e, but I'm pretty sure the hireling/henchman difference is the same)