The GM can assign stats to NPCs as long as those stats fall within the possible results of having rolled them. I could just decide, for example, that Joe the (Human) Blacksmith has St 15, In 8, Wi 12, Dx 13, Co 11, Ch 10 without breaking anything, as those stats fall well within the roll-able range for a Human.
Having run various PC-v-NPC battles where the NPC foes were of similar calibre to the PCs (e.g. the PC party was fighting another adventuring party), I can say two things from experience:
That's the 4e synthesis. The premise of universal stats in 3.x was precisely the opposite; that I might very well care about the bartender's Profession (publican) skill, or his Listen check, or I should be able derive from a game mechanic how likely he is to hit the fighter when a brawl breaks out.
The ones without class levels are commoners of the species.
Anther instance when the barkeep's stats and class levels become very relevant is if the PCs are for some reason trying to recruit him or in some other way get him to help; which in low-level play can happen fairly often IME.
I agree that NPCs are often 'sort of like' certain PC builds. I think there are a few reasons why. Logically both fighters and militia are martial weapon using paradigms. Why wouldn't they be similar in some ways? Nobody is claiming that it's not logical and perfectly reasonable to suppose there are 'fighting styles' for example that a PC and NPC might both know. Despite people complaining about all the differences a 4e fighter and a 'guard' stat block can easily have an almost identical power! Some creatures do things with weapons that a PC can't exactly emulate, but usually you could get close enough to be indistinguishable. Often it's just a sub-optimal approach.
Why would any NPC caster hold back? Try this in 3e or 4e, In 4e the NPC side wins hands down 3 out of 4 times. It may be more random in 3.x but that's not good either! 5e is probably a bit more workable, but even so NPCs are free to burn any daily resource like spell slots right up front.
In 1e, an NPC "character sheet" is a brief stat block: level, ability scores, hp, AC, alignment, spells, equipment - but bear in mind that a PC character sheet is barely any more than that: you might add languages and encumbrance. While there are plenty of examples of deviation from any standard formula ("the minstrel can cast Magic User spells at the fifth level of ability"), there are also plenty of examples where all of this data is presented relatively coherently and consistently, and the NPCs follow the same rules as the PCs.
That assumes the NPC casters are going to nova while the PCs don't. However, the NPCs - just like the PCs - would assume they're going to win and thus be concerned in-character with what happens next after this battle is over, and in light of this might conserve resources just like the PCs do.
And in my game they're way more interruptable than any of 3-4-5e have it.
Believe me, I know chapter and verse of 1e characters. Yes, if you ignore a lot of stuff you can create fairly brief sheet for a fighter, but it will omit a lot of important stuff unless they are really low level, plus what is gained for such an NPC? All you actually care about is level, hit points, AC, etc. Nobody cares about it's CON or whatever.
It may well depend on which G3 you have? Mine is the original standalone module. I didn't go through them in detail, but the casters seem heavily standardized and omit a lot, so we can't really say they're legal or not. Yes they have some bits of info not found in MM stat blocks, but rules don't actually exist in 1979 era 1e for Drow anyway, and even later the rules were inconsistent and in most sources differ from monster Drow in various ways.
