Tiddies
....on a more serious note, there is actually a logical reason for going into battle with minimal or no armor - that is, exhaustion. Armor is great, and it isn't actually that heavy when worn properly; however it *will* get hot and sweaty, and drain your stamina. If you become exhausted in a battle, you die. Wearing minimal armor, or armor only in key locations helps alleviate this issue, and grants the warrior greater agility and stamina during a fight.
Of course, anime armor's sexual dichotomy is ridiculous nonetheless.