First: This was a really neat idea. Thank you.
Second: I'd also agree a scaling accuracy buff would not be a bad idea for this. It seems like most of the comic book situations you based this concept on included that.
Third: I tested this with a brand new Sonic/Eng blaster I rolled up, my Eng/Dev blaster based on my I4 test build at level 24, and an Elec/Eng blaster at level 19.
From around 1-8 (DJ Boom's (BOOM, BABY!) current level), I found myself doing really well as long as I started fights at 50% health. This won't be a good tactic after I start accruing debt.
For my midline guy, I found a solo Heroic mission in the Hollows (defeat all Trolls in cave, from Julius) to be where I started using it less. It was just too risky and didn't last long enough unless in a one on one fight, though it did save my bacon when an Ogre I ran into outside the mission decided to try to eat me.
For my 24 blaster, it was not a factor. In a street hunt against Family in IP, I either held my own without the definance meter ticking anywhere beyond 10%, or by the time it had ramped up, a Capo was scraping my armor off his boots.
Suggestions:
Start the defiance ramp higher. 60%? Also, the defiance meter needs to be easier to read. I'd suggest a bright red or yellow that will stand out against the UI, and very possibly flash or pulse while Defiance is at cap.
Defiance should last longer. Either let the capped defiance last for a certain period of time (10 seconds?) or make it recede slower than it ramps up as long as the blaster is using combat powers.
Buff accuracy on the same sliding scale as damage to HP, but don't make the ramp as extreme. I'd say cap the accuracy at 200%.