What's the most important part of the trinity?

While the structure of the jokingly-named "holy trinity" in MMOs -- tanks, healers, and damage dealers -- isn't universal, it's certainly common enough to be recognizable. Even in games without pre-defined classes, such as Champions Online, have a group structure oriented toward the three roles.

But you can't have classes without a struggle between them, and there's always a discussion in relation to the trinity about which part is the most important. Tanks point to their relative scarcity and the pressure of their job, Healers point to their relative scarcity and the importance of their job, and DPS points to the fact that healers and tanks can't keep their stunts up forever if nobody is killing the target.

So where do you fall in this debate? Which part of the trinity do you think is the most important to a group's success, and which one is the most superfluous? Do you think that (by astonishing coincidence) the role you play most frequently is the most important one, or do you think it's lower on the scale and you play it for other reasons?