^This. Also, there are two potential aspects of PLD where casting Clemency could be considered necessary as part of a PLD's skillset (or at least keeping HP up to maximum) - Divine Veil needs a heal cast on the PLD to activate, and although a lot of PLDs like to save it for later (such as a battlefield wide AoE), given the effect lasts for a fair while if no damage is taken, it can be just as beneficial casting it near the start so procing it via Clemency is definetely acceptable (not to mention DV's cooldown isn't too long so you could potentially use it several times in a fight).
The other potential requirement for Clemency as part of another PLD ability is for Spirits Within which on paper at least does more damage the more HP you have (so theoretically at full HP you do the most damage), however this isn't nearly as effective in reality as Spirits Within has a long cooldown and thus cannot be spammed (if if could, then that would change everything). Also it was my understanding the ratio between current/total HP and damage potency with that particular ability caps around half current/total HP so it wouldn't be quite so critical to keep HP up.
Also Clemency has a trait of healing the PLD casting it by half the normal amount if it's cast on a party member, so it's clear SE actually wants players to make use of it as a pseudo-healer, (which is not surprising given towards the end of 1.0 PLD pretty much was a backup healer due to the only actual healing class/Job in the game being CNJ/WHM - SCH and AST did not exist back then, so PLD's healing skills made sense), as why else would it have such a trait if they did not want you to heal your party? Of course it's entirelly situational, but dismissing it's use outright only handicaps the player in the long run.