I was just in a random duty, Halatali, and the tank was absolutely horrible and he had a dps friend so he could not be removed. We had gone through 3 healers in about 15 minutes and the fourth was about to leave so I just left rather than waste my time any more. Why should I get the same 30 minute penalty as someone who leaves at the start? A healer roulette had just started so I figured I'd do something productive with my time.
Also, if a healer leaves you really should be able to opt to change classes to healer and look for a dps instead. If I had had that option I would not have left the run above since I could have dealt with the bad tank.