It's a high minded goal, but it's a lot easier to say "create a non-shitty AI that doesn't need to cheat" than it is to implement. Even if we ever do get to that point, developers may still need to make concessions about balance in player/enemy just for the sake of gameplay. If the DCSS was as intelligent as a human:
You would be virtually unable to flee from a powerful enemy, or an enemy that you are not ready to fight, without use of tools (high speed, blink, teleport, perhaps invis). You can't just use swiftness and run up the stairs when the enemy is 2 tiles away to save yourself. He saw you and he's going to follow you, unlike now when he's prevented from going up unless he followed you up from an adjacent tile.
The enemy will spend its time using its strongest action possible, until such resources are exhausted. Priests and spellcasters won't spend their time idling with empty spells, they will be spamming their strongest spell on you all the time (until they run out of mana). Uniques with wands will blow through those until they can't any longer. The exception to this would be if the AI feels it has more intelligent action to take (maybe buffing their allies or using a spell with a longer range). Undead would be pretty much screwed, because dispel undead is the only thing they'd hit you with, if they have it. Breaking LoS and letting them come to you won't help - if they were as intelligent as a human, they'd not just follow you around the corner, they may circle around. Even if they did you still have to contend with their most powerful ability, which is likely to be wands/dispel/smite/etc even in melee.
The goal of the game, as far as the AI is concerned, is to kill the player. Since the game ends when that happens, they aren't bound by the same rules as far as resource conservation. An intelligent human playing in this role will dump everything they can at you, because they win if you die, but they're 'just' a hurdle for you - you have more to do after.
Anyway, a truly intelligent AI is a nice goal for a computer game, but needs of compelling or fun gameplay trumps that.