A big thing I see lots of wars forgetting is that Defiance offers no additional mitigation. With proper cooldown usage (and good gear), you can go into deliverance on trash pulls and Decimate your heart out with berserk and inner release, with good results. But as with everything, use your own judgement.
For dungeon bosses I exclusively tank them in deliverance after pulling in defiance with tomahawk->equilibrium. If your gear isn't good you might need to use a butcher's block in defiance, but prioritize putting up storm's eye.