A thousand times this.
When DPS are focusing on whatever the tank is attacking and leaving a pile of trash to attack me, I am
going to kill the trash or I am not going to live, and dead healers do even less healing than DPSing healers.
I'll add another thousand times to that as well, unless dps and tank are doing their jobs your healers not going to get a chance to heal you because they'll be too busy trying to keep themselves alive. Just so people don't think think i'm biased i'll admit there are healers/dps and tanks out there that have no clue what they're doing (and often then blame everyone else but themselves for the failure).
To the original point though it entirely depends on circumstance, what strength mob your fighting, what level, what gear you have, what classes your group's made up of. Example i ran a lvl50 FP the other week with a tank and dps companion and two heal players off-dpsing. That worked far better than the useless tank and dps players we started with who didn't know what target priority even meant when we told them what they were doing wrong. Probably didn't help that the guardian tank didn't have soresh form up, was guarding no one and had several k less health than me a squishy seer sage with nothing more than tionese level gear on /RantOver.