Clemency is part of what makes PLD the actual best tank in the game. I've had countless situations where I've been able to use that and other skills to help people hang on and get a fight done.
Cover is underrated, btw. There are a lot of effects in game that won't go through if the target takes 0 damage.
Some folks are too hung up on this idea of everything having to do DPS. Maybe they should stick to that realm of job to begin with. At the very least, look at playing one of the other tanks with more DPS in mind, like GNB.
To any that say a healer's output will be enough, you haven't seen what some of them are (not) doing these days. They're trying to favor DPS in dungeons because they're told that's what works in raids. I've seen all kinds of missteps from SCH not using soil to WHM burning afflatus stacks IN BETWEEN PULLS for that one lily shot. I was working on my GNB's tank you achievement and I did most of that with a friend who I can trust to heal me. I've now shelved the job until next expansion.