I have actually noticed a slight increase in damage a few times with Defiance, but it was minor and I was on the run. When you compare Critical Hits with Defiance, there just is no comparison. You never know when Critical Hits are going to happen, but when they do go off they can be substantial, sometimes one shoting an enemy. Defiance on the other hand, only comes into play when you're on your deathbed, a place nobody wants to be. And the fact that Defiance starts to have an effect at that point isn't really useful, unless you are on a suicide mission. If the effect kicked in at 50% instead of 10% health, it might be worth something. As it is now, I simply ignore it.