Thank you for the responses.
All make sense to support keeping Defiance active all the time while tanking. I was trying to get disruptive a bit and also like the idea of a tank having to switch between off/def stance to actually tank.

Just an example of what I had in mind:
Base HP = 4000
Defiance HP +25% = 5000

Let's say you're down at 1000/5000, you could drop Defiance and get your hp at 1000/4000 if it took more than 10sec (defiance recast time) for the healer to get you close to the 4000HP and then re-activate Defiance so you don't get over healed. then maybe during those 10 sec, you'd be able to do enough damage with your true offense potential to self heal more than the +15% heal.
if I assume that a Defiance WAR does 100 DPS, and hoping that DPS would raise at 130 by dropping it, over 10 sec you'd get 325 HP more (25% of 130 over 10 sec) than in the normal scenario.
Pretty negligible but I don't have a feel of how much the +15% heal would provide over the same period