Here is what people don't understand.

Job design is about overall balance.

Yes, WAR does not get immediate mitigation purely from stance dancing to Defiance.

That's irrelevant. Completely irrelevant. Why? Because Defiance doesn't exist in a vacuum. Neither does Deliverance. If you want to view WAR stances in a vacuum, then you should also look at stuff like Inner Beast, Fell Cleave, Decimate, Equilibrium, etc. in a vacuum. Defiance might be worse than Shield Oath when removed from its context. But, if you view IB, FC, Decimate, etc. with the same standards, WAR skills would deserve to be nerfed into the ground because they are so much better than what PLD and DRK have.

The point is that Defiance and Deliverance have to be weaker because you have to factor in the value of stacks and stances allowing otherwise completely overpowered skills like IB, FC, Decimate, etc. to exist. There might not be an upfront gain in mitigation when activating Defiance, but you gain access to a number of incredibly strong skills that only exist because of your weaker stance. That's a persistent strength of the stance just like how mitigation is a persistent strength of Grit and Shield Oath. Imagine PLDs / DRKs with IB, FC, and Equilibrium. It would be ludicrous.