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.
Any good team will have a tank that will always swap guard and taunt. You can't simply have all your dps FF one of their dps while ignoring the healers. You have to either CC healers or have one dps stick on them to interrupt.