Equilibrium gives a big heal instantly in between GCD and you don't need to stop attacking. Took too much damage? O shit equilibrium can save ur ass, and it cannot be interrupted. Clemency you have to stop attacking to cast it (3 seconds) and you run the very real risk of getting it interrupted while tanking.

Cast interruptions are a function of physical damage dealt as a % of HP. Of course a Titan HM autoattack isn't going to interrupt you, it hits like a wet noodle. Try casting PLD stoneskin in T13 with no echo in i110 gear, every auto will interrupt you. In any content that matters, if you're casting Clemency/Stoneskin while tanking a hard hitting raid boss expect to be interrupted by auto attacks.