Tanks do this; Big hit - Use thingy to reduce Big hit! This is what all 3 tanks already do effectively. If they couldn't do this they wouldn't be tanks. Adding gimmicks to "Use thingy" does not inherently change this outside of making it require procs and we all already hate RNG (Parry Block Reprisal Shield Swipe say hallo). Dark Knight uses the thingy and the other thingy, Warrior uses the thingy and expends the stack thingy, Paladin uses the thingy maybe uses the stoneskin thingy, and has a useless heal thingy that gets interrupted mostly.
Auxiliary functions of these tanks become; Warrior does dps - buffs tank(s) & Ninja damage - reduces raid wide damage taken, Dark Knight does dps - reduces magic damage taken (if you don't have a Monk) and reduces raid wide damage taken if it parries (force that main-tank role on ya), Paladin does less dps - reduces physical damage taken anddd it's extra mitigation is mostly hiding behind a shield (again force that main-tank role).
Why is that ^ an issue - Paladin and Dark Knight are always second fiddle (or at least need to be the one to be getting hit in the face) to Warrior when it comes to a raid. Always. Didn't bring a Warrior? Why? Go get a Warrior. Put your Warrior in the OT slot and enjoy having more damage, and more mitigation. Do it! Do it all the time - every time because DRK and PLD aren't good at off-tank roles because they need to get hit in the face for their shiny extras to work.
Hell even if Cover sucked less it'd need to also allow you to block Covered damage so you could use your other junk - like Shield Swipe since you don't have a TP factory in your butt...
Like Warrior.
And even then - DRK MT with a WAR OT would be better unless the content suddenly was pulling gimmicks like Bahamut's Ahk Morn dookie-dumper followed by Nael's back-scratch attack or whatever.
It's really a balance issue between WAR vs. PLD/DRK; WAR always gets a slot because it reduces both types of damage, and increases it's own damage, and the overall raid damage potential. Oh and it does the most damage too.