I've played a fair amount of M2TW, and I never noticed the AI spamming their cities full of free upkeep units. I don't think the AI is aware of their free upkeep ability, and simply evaluates them as standard troops for their cost and field upkeep. I think it's much more likely that if the Klerouchoi Phalangitai cost a lot more to maintain in the field, it's possible the AI will dismiss them as not worth the money, and end up not training any at all.