I'd have to agree that defiance is of limited value at the higher levels. It does work out to a huge string of "ifs" for it to help at all - i.e. if you happen to take damage to drop your HP down to the right level (without dropping too far) and if you are only facing one or two baddies and if you happen to have a power available in that golden moment before you die and if you actually hit the baddie. It's not that defiance is particularly bad, it just seems kind of pointless to put so much work into something that you would want to avoid using as much as possible.
The other part of this is something I think CoH is doing an increasingly poor job of - information to the player. It seems like we're getting too many things being represented to us visually that require us to take our eyes off of the main battle area. Why not change the sound of the attack when defiance kicks in? When you're flirting with death to begin with, glancing up to see where you defiance meter is can be a very bad idea. At least make some kind of change to the damage numbers onscreen. For instance, if the normal power would hit at 50HP and defiance added an extra 10, instead of just showing 60 over the baddies head, show 50 +10. Seeing concrete evidence of it's effect would help people to feel it's a useful power.
