I think people don't seem to understand that Defiance doesn't give any damage reduction. If a 10k tank buster hit a warrior in defiance with 10k hp, he will die instantly while a PLD or DRK with 10k hp will only get hit for 8k in their respective tank stances and if a warrior switched from Deliverance to Defiance he would still be sitting at 8k hp which would still kill him. This is without taking into account and defensive cool downs. This a point right here validates why warrior's tank stance is more flexible as this one simple act of pro and con necessitates why it needs to be ogcd.