I should have mentioned AD&D's common descriptive language didn't include the 6 abilities. Those were PCs/NPCs. Monsters did get an intelligence rating, but it was given as text, not a numeric value.
Although the intro/glossary for the MM does give a translation of that text into a 2 or 3 number range (I remember, when I first read it, being struck by the definition of Average intelligence as 8 to 10, which is below the 3d6 average).
As for interacting with the action resolution mechanics... well, ability scores didn't do a whole lot of interactin' back then.
This is true. I remember that the DMG has a table for calculating a creature's WIS if needed for psionic (I think?) combat.
Roger Musson, in his White Dwarf article "How to Lose Hit Points and Survive" (the first version of Wound/Vitality that I'm aware of) gave a chart for calculating a creature's CON score, which his system required. (That said, why don't creatures need to roll system shock checks when they get polymorphed, hasted etc?)
It's strength was reflected in it's damage dice and/or size. And so on.
It's as inelegant as it's not-cumbersome. I'm coming around to appreciate that kind of design.
I find myself a little bothered by its oddities. An ogre, for example, is 18/00 STR and attacking with a club or spear, but from memory does 1d10 damage (average 5.5), whereas a PC with that STR (perhaps acquired via Gauntlets of Ogre Power) will do 1d6+6 (average 9.5) with the same attack.
Now I'm not saying that this is per se objectionable - that sort of difference is fairly ubiquitous in 4e, for example - but in AD&D it still strikes me as a bit odd, because in other ways AD&D tries to be more concrete than 4e about what stat bonuses, damage rolls etc mean.
Those oddities aren't a deal breaker - RM monster stats have similar oddities (but also have the virtue of being non-cumbersome), and I worked with them for many years. But I do find myself liking 4e monster stats better.
For stats that are both elegant
and non-cumbersome, I think it's hard to go past RuneQuest.