I haven't had my account active for a long time, so I gave things a go over the free long weekend.
From what I can see I agree with almost everyone else, defiance does not appear to do anything. If your health is below 40% you are probably taking too much damage too quickly for it too help. If you are fighting correctly your health should only dip below 40% once and a while.
Mabe if it kicked in at a different % you would notice it doing something. OR if it was a flat high bonuse at at 40% it would do something you could see.
I just play as if it isn't there, it doesn't seem to improve or detract from my previous play style.