As a tank, when my HP gets low I have to assume the healer is incompetent and use clemency on myself.
HP low and I assume healer incompetence: if I’m right I live, and if I’m wrong I also live.
HP low and I assume healer has it under control: if I’m right I live, and if I’m wrong I’m DEAD.
Only assuming incompetence guarantees survival. So please don't let your tank’s HP get low!