One of the things to remember about AD&D is that it was designed for a markedly different style of play than we typically see today.

Specifically, it was designed around an open table with a shifting number of participants, each of which may have multiple characters.

A lot of the rules that don't make any sense from a more modern "we're the party of adventurers on a quest" style of game work perfectly with those assumptions.

As an example: association restrictions. In the "single party" model, playing a Paladin means nobody in the party can play an Evil character, which locks out certain classes. Which kind of sucks. But in the "open table" model, playing a Paladin means that nobody can play an Assassin *that session*, which is an entirely different situation.

Similarly, maxing out on level as a Halfling Fighter (keep in mind that there was no presumption in those days of a campaign meaning everyone gets to level 20) in a game like that just meant that the Halfling hung out with appropriate characters, and the higher level guys hunt out together.

I personally think that old D&D works best around those types of games anyway, so I probably wouldn't use it for "The Big Heroes on their Big Important Quest" style of game (I've got other games that work better for that). But if I were, I'd probably take a very critical look at mechanics like level limits, association restrictions, training time, etc., as they really are kind of counter to the "Big Heroes" model. But then, arguably, so are things like level draining, etc..