Well uh, yeah. They aren't. I've been running the new dungeons on Dancer and, with the exception of like one boss, if the healer drops dead but the tank, I and the other DPS are still standing, we had zero issues surviving. But that's not what the OP is saying. The OP is saying that a tank trying to solo the boss from a high HP percentage while everyone else is dead is wasting everyone's time, which they absolutely are.