After playing my level 50 energy/device blaster, I didn't notice any difference due to defiance. I agree that, as a blaster, you run before life gets much lower than 40% at higher levels so the power is not entirely useful at those levels. Plus the bonus wasn't discernible. Since it is an automatic power it doesn't hurt but in it's current form it won't solve any of the blasters' problems. Respecing and taking more defensive powers seemed to help more.