You seem to forget Grit itself is a defensive ability. Defiance isn't on it's own, and requires healing to be any sort of thing other than a nerf to the warrior using it.
Sure you can blow 50 Beast Gauge on Inner Beast for some defense on WAR, but you don't need to spend your blood on Grit like that. THere's no equivalent to the defense of Shield Oath and Grit for a Warrior.

This is the main reason Defiance and Deliverance are oGCD while Shield Oath and Grit are not.