I'm a number cruncher. An extrapolater. I may not min/max to it's full extent, but I'll do my damndest best to *think* about it. There is no substitute for experience, but nor is there any substitute for the ability to foresee a situation before experiencing it. As such, I have not tried Defiance at ALL on the test server. However, I have more than ample experience as a blaster, and while it is *technically* true that Blasters die a lot when their enemies are pretty close to dead, increasing their damage when they're about to die will have little effect. Why? Because when they're about to die, it's not because their next shot didn't do enough damage, it's because their previous shot didn't do enough damage, or because they're in some way being controlled. Holds, immobilizes, heck even the knockdown, the blaster is a couple of seconds from an unlucky death at any given time.
No blaster with an ounce of common sense is going to hang around when he has 40% or less health left. Regardless of the HP increase, that's simply one hit from death. I'll grant you that Defiance certainly doesn't hurt you a whit, and I'll even grant you that over the life of a blaster, Defiance is probably going to save his life more than a few times. I'm a blapper, and I've wagered my life on a melee hit more times than is really wise. But Defiance is still an incredibly dumb power. Stupid and unuseable in any tactical sense.
The idea of a "desperation attack" is innately an appealing one, but it simply doesn't hold value to a blaster where a chance of coming out victorious pales in comparison to the debt incurred if you die once. If Defiance were so powerful that it saved your life 4 times out of 5 when you were about to die, a coldly rational blaster would still run away in the vast majority of situations because it takes dozens of victories to equal one death.
Shrug, I'll take it, it's still *something*, but it's current incarnation seems like a waste of a good idea. As other have said, I'm sure a tanker would appreciate the power much, much more.