The AC rating for armor is incomplete. Always before you lost dexterity bonuses for wearing heavy armor. But, if rooms in light armor were caught off guard or flanked they also lost dexterity bonuses. We have a problem with a tank in chain mail always having the same AC as a rogue in leather all the time. If someone shots an arrow at a rogue who doesn't see it how can they react to it. If in a hallway fighting two or three monster , you really can't move out of the way of one attack without exposing yourself to the others. Why buy the heavier , slower armor if it doesn't offer better protection all the time without accounting for your reflex.