You ever played on an 8 man rated team where you can get 4 good DPS on a single target? No healer(s) can keep him up, unless there is also a guard and an AoE taunt to help.
But again, it depends on respective team coordination levels. If you can only get one or two people on the same page, stunning a healer first can work great.
under focus fire, many classes traditionally on the front lines and thus most exposed have at LEAST a 20% DR cooldown, a 15% DR adrenal. that's an additional 35% DR, with at least 10% more from sorc buff/armor including shatters. so 45% DR, + a guard (not including taunts) means 1/2 incoming damage mitigated to almost 1/4 total damage incoming. essentially reducing the efforts of 4 DPS to the output of 1 DPS. easily healed through. throw in a taunt, and its laughable.