Looking at the description of the intelligence attribute itself, it seems to have three main uses:
- Learning additional languages
- Casting high-level magic-user spells
- learning listed spells/maximum and minimum number of spells per level
The languages issue is rather an odd one, as non-human characters generally already start off with a boatload of languages, and only get the chance to learn one or two more if they have an exceedingly high INT score.
Being able to cast spells of 5th (INT 10+), 6th (INT 12+), etc. level is certainly something a magic-user needs to have, and at least gives an incentive for players not to try to get cute and put their highest roll in something that has more immediate applicability (like constitution for hit points or dexterity for armor class).
The last one; the chance to know listed spells, and the minimum/maximum spells per level is simply a non-issue. I've never heard of anyone actually using those cockamamie rules. (Basically, each M-U character is expected to roll for each spell listed in the Players Handbook; if they blow their roll, they can't ever learn the spell in question. Ever. And someone with, say, a 15 INT would only be able to ever learn 11 spells listed in the Players Handbook. Ever. And even then, it's no guarantee you'll ever find the 11 spells you are "able to learn.")
Unfortunately, that useless rule is also the one that gives some heft to the INT statistic. Without it, in terms of day-to-day use, you have languages. Strength gives you all sorts of strengthy things to do like opening doors and melee combat. Dexterity gives you an AC bonus and missile combat bonuses. Constitution gives you permanent HP bonuses. Even charisma, if utilized properly, allows you to get and keep henchmen and hirelings. But intelligence only gives languages.
My question, and I am actively soliciting your opinions on this, is whether you agree with me that intelligence, as currently described in the AD&D rules, is really underpowered (or perhaps "underutilized" is a better word) than the rest of the abilities, and if so, what to do about it?