Tank IDENTITY

[fulminating]

And that's where self-sustain comes in. I can think of many ways of doing self-sustain on a tank. PLD's classic spellcasting approach with Clemency has its issues because of the opportunity costs in cast time and resource costs, which makes it a dps loss. But that wouldn't be an issue if you had a way of generating a swiftcast proc that lets you cast healing and defensive spells instantly and for free. You wouldn't need a lifesteal effect built into Holy Spirit/Circle. Need some spot healing? Just weave in a Swiftcast Clemency. Done.
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Clemency is a spell, no matter how hard you swiftcast it you'll still not be doing weaponskill/spell damage that gcd. Changing clemency to an ability probably wouldn't help much either because you'd now have a 1000/500p heal on demand which generates the opposite issue.