I really fail to see the issue with letting a tank drop below 50% health. Damage in this game comes at incredibly predictable intervals and healing is based entirely around oGCD heals that often benefit immensely from allowing the tank to drop to lower health (Excog, Benediction, etc.) Sure, if we die on the first wall to wall pull because the healer doesn't know what they're doing, then that sucks - but I've seen that happen maybe once in the past 100 dungeon runs, whereas me trusting the healer has let everything go much more smoothly the other 99.

Healers doing DPS isn't them playing badly or messing around with your life, it's just them taking full advantage of their kit and playing in the way that the game is clearly intended to be played.

(That said, clemency can and should be used if the healer is dying, if you're familiar with the healer's kit and you can clearly tell that they're out of oGCD options, or you mess up and take needless damage.)