This is a terrible power...kind of like anyone other than a tank (or maybe a scrapper) getting skills from the presence power pool...its a death wish. Sure it may help below lvl 10 where u solo and can take a couple hits at low health...but was that realy the intention for this? To give lowbies a hand? There are no practicle uses for this in a team or even solo at higher lvls. If you are a blaster and your health is low...it probably means you have drawn too much aggro, doing more dmg will only INCREASE the aggro! You will die, because its only a rare instance where it is just one enemy kicking your butt and dmg buff may actually save u. While defiance may help kill that one enemy...there are about 12 other guys who are pissed at you and aren'y gonna sit around and let defince help u. If you want a practical power give blasters the full buff at 40%, cause chances are that they won't be alive much longer anyway.
Honestly...do the devs even play this game?