No, it isn't due to the diminishing returns on stacking mitigation and relative amounts of damage healed. A 10% mitigation ability used on a tank is in actuality only a 7.5% increase in mitigation, and the "insignificant" amounts of healing/shielding NF and HoS+Brutal Shell generate outside of burst phases are much more significant on Healers and DpS. The mitigation gained from using these abilities on your Co-Tank is less than the mitigation gained from a full Tenacity build.
No, there is no piece of content (even in the Ultimates) that needs one tank to use their abilities to protect the other tank. The whole mentality of the use of these abilities on the other tank is built off of the use of Cover to cheese various mechanics in ARR, HW and ShB; especially the uses of Cover in ShB to turn it into an extra Rampart on the other tank. All using these abilities on the other tank really does is make the Healers overheal a bit.HoS, Intervention, TBN and NF are only really useful for the other tank because they are constantly subject to damage and especially high amounts of damage in the case of a tankbuster. Yes, there are cases where you can use these on a non-tank and it will help ( Covering preys, TBN a DoT's damage, etc.), but they only really help out a cotank as it allows mitigation sharing and support.
Also the current Savage design forces both tanks to mitigate tank busters at roughly the same time (or have one tank Invuln the multi buster).
PoA, Reprisal, D.Missionary, SiO, HoL and DV are all full party mitigation. Intervention, Cover, HoS, TBN and NF are all single target mitigation.Tanks have better ways of shielding the party ( PoA, Reprisal, D.Missionary, SiO, HoL, DV).
First off, I never said that. You should always be mitigation tank busters with one of your Primary mitigation cooldowns (Rampart, the job's 30%, or their Invuln).Just because Tanks have high natural defense and mitigation is no reason for you to help out your cotank ( again, by your logic I should eat tankbusters naked with no defense CDs because " my fending armor gives me high enough mitigation who cares"). By helping the cotank, you in turn help the healers by reducing incoming tank damage strain.
Second, all 5 of the single target protect other abilities come at the cost of each tank's personal mitigation when used on others: Intervention and Cover share resource gauge with Shelltron, HoS and TBN can only be used on the tank or a party member never both at the same time, and RI and NF share a cooldown.
As I said due to current content design you are not helping with the tank busters because you are getting smacked with one at the same time/ consecutively. You can't use HoS, TBN or RI/NF to mitigate the damage of Spear of Paradise, Shadowflame, Rip Current or Stone Crusher on both tanks.Does this mean you can't use it on other ppl? No. But these buttons aren't purely designed to shield DPS or healer, they're mainly meant to help out the cotank. You can share mitigation with intervention, give a fat shield with TBN, give small mitigation and shielding with HoS, and small mitigation and some healing with NF. And it benefits the tank since they're taking autos and can help with mitigating tankbusters.
Auto Attacks have for a long time been superfluous and any increase in mitigation will not reduce the amount healed on the tanks. If it did, Tenacity would be considered a good stat.