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.
Wow, you must play horrible healers. 2 (even better, 3) healers can keep someone up versus 4 DPS just fine when you leave them alone.