Unchained doesn't increase attack power. Defiance is merely a flat 25% reduction in damage output.

Unchained is an extremely situational use DPS boost on a very long cooldown. Frankly, it's one of the worst skills in the game. There are only 3 situations you should ever use it: 1) Phase 1 of fights to get a significant lead in Enmity when the boss doesn't have any increased killing capacity (i.e. Phase 1 titan, 0 stack Caduceus after split, etc.), 2) Burn Phases when bosses don't put out big amounts of damage but something has to die quickly (i.e. Titan Heart phase), and 3) Times when you are off tanking and too lazy to toggle off Defiance for whatever reason.

#3 is really iffy though. In general if you want the Unchained damage buff and aren't tanking all you have to do is turn off Defiance. It just sets you back 20 seconds in building Wrath stacks, but that shouldn't matter if you aren't taking damage. One situation though might be in a tank swap situation where you want to have 5 wrath as soon as you pick up the boss. In that case it might be worth it.