An important sentiment I've heard many times on the gamedev.net forums is that machine learning techniques are usually too difficult to tune in order to make the agents behave in the way that the designer wants.
Game AI is about providing an illusion of intelligence and about providing a designer-crafted experience to the player. So it often makes sense to cheat in order to achieve this.
Having said that, some games really do benefit from smarter AI. For example, real time strategy games sometimes do (eg Supreme Commander AI mods, AI War Fleet Command etc) and often first person shooter bots as typical dumb bots are simply not as fun to play against than real humans, although you mostly want the bots to have some high level tactics ability (not perfect aim or such), which is why the game FEAR is often applauded for having fantastic AI (it uses Goal Oriented Action Planning to produce high level tactics such as flanking). But most games don't go very far at all.