Should shield blocking be effective against magic damage?

[Reynhart]

Which specific cooldowns are you comparing, and what warrants comparing these abilities in isolation instead of looking at the kit as a whole?

Skills that only work on a specific type of damage.
The "core" mitigation skillset (Rampart, Sentinel, Shadow Skin, Inner Beast, etc...) is mostly balanced between the three tanks.

But if you put a PLD in a very heavy magical fight, like A1, it suddenly loses a lot of skills, since the shield is useless.
DRK, being the magical tank, really shines with that type of fght, mainly thanks to the powerful and frequent Dark Mind. But, if you put it in a physical fight...well, it still holds fairly well if it can properly time Dark Dance and Reprisal.

WAR has around the same issue as PLD, even though it only loses one skill (Raw Intuition) during magical fights. Yet, being able to use Inner Beast very frequently and being able to heal itself pretty well makes it a strong tank in every situation.

That's why some people are suggesting PLD's shield should be used to block magic with the proper cooldown. And since Sheltron was shown to block a Fireball in the HW benchmark, it makes it a good candidate.

[Lyth]

You can mitigate A1S on PLD with Sentinel, Hallowed, Sentinel. You can throw in Rampart to mitigate a few cleaves along the way. The main issue with this in a progression situation was needing to wait for Hallowed to come off cooldown after a wipe, but that's a non-issue now. There were no limitations on your ability to tank this on PLD, and interestingly enough, the world first clear was done using PLD/WAR.

Unlike Sheltron, DADM is not free. It costs you DA, and by extension, dps. It's also the reason why Sentinel is objectively more powerful than Shadow Wall. Interestingly, if I choose not to spend the DA cost, the combination of Shadow Wall (30% DR) and DM (15% DR) gives me the same damage reduction as Sentinel on magical attacks (0.7 x 0.85 = 0.595, which is about 40% DR). Whether I pay that extra cost will depend on whether it gives me more uptime out of stance, which in turn depends on gear and the amount of damage I take. There are a few ways that I could do this. I could go SW+DM, LD, SW+DM. Alternatively, I could go SW, LD, SW and use DM and Shadowskin to eat cleaves. It's not that I'm mitigating something that a PLD cannot. The extra cooldowns just give you more freedom out of stance.

Reprisal and DD aren't used in isolation to mitigate tankbusters, and are always used as adjuncts to other abilites. On average, PLD block procs on their own account for an average damage reduction of about 6% over the other two tanks in a physical fight, before taking RoH into account (there was a reddit thread earlier this summer which collected the data on this). Reprisal is 10% damage reduction with a theoretical maximum uptime of 66% (in practice, this will be less), which on average just about offsets the passive contribution from block procs in single target.

In both physical and magical fights, WAR has access to ToB. This isn't quite as good as Rampart on a single hit tankbuster, so you would likely pair this with IB, but it's still a versatile option.

Sheltron is free, and is not stance locked. It mitigates more damage in a single hit than IB. If this extended to magical attacks as well, it would be incredibly unbalanced. You probably wouldn't require any additional cooldowns on most fights.

[Reynhart]

Sheltron is free, and is not stance locked. It mitigates more damage in a single hit than IB. If this extended to magical attacks as well, it would be incredibly unbalanced. You probably wouldn't require any additional cooldowns on most fights.

Incredibly unbalanced as in "You would chose a PLD for every setup even though it does less damage than WAR and DRK" ?
Mitigation is not the only thing to consider in tank balance, especiallly when it didn't help PLD for the first 6 months of Heavensward.

A little not on Dark Mind, though. Yes DM+Shadow Wall will "only" give around the same mitigation as Sentinel. But, comes the next big magical attack, there's a good chance that DM will be available again to pair with Shadow Skin, where as PLD would only have Rampart ready.
And if Rampart or Sentinel is enough to mitigate a tank buster, you might also use DA+DM when PLD would have to use HG, thus saving Living Dead for real shitty situations.

[Lyth]

Unbalanced enough to make me reconsider having Shield Oath on my hotbar. When you play a tank, how well you dps is generally a function of how well you mitigate. Defensive cooldowns that let you stay out of stance function as offensive cooldowns.

You can skip using LD if you want, sure. But any CD that is held for longer than its recast is a waste. This translates into a dps loss. Hallowed is the most powerful tank cooldown in the game. Why wouldn't you want to use it in a fight?

[Reynhart]

You can skip using LD if you want, sure. But any CD that is held for longer than its recast is a waste. This translates into a dps loss.

What ?! That's nonsense...

You don't pop long mitigation CD haphazardly just to skip tank stance for a few seconds when you know a tankbuster can kill you. And keeping the "oh!shit" button for "oh!shit situation" is pretty useful

[Lyth]

As a tank, your job is to anticipate damage patterns, not react to them after the fact. When you don't know a fight, then you're forced to play defensively and hold on to cooldowns because you don't know what happens next. As you learn the timings, you stop playing reactively, and you can work on being consistent. This is much more beneficial to your healers than taking unnecessary damage because you are holding cooldowns in reserve, and then blowing them all in a panic when your health is in the double digits. There are no surprises when you have everything choreographed in advance.

You can think of tank stance as an on-demand cooldown. The main cost is the damage penalty. If you're in tank stance, it's either because you need the enmity generation, or it's because you need the mitigation. Your dps is directly dependent on how low your stance uptime is.

If the boss is just using auto-attacks, there's usually no reason for you to be in stance. Moderate damage attacks and cleaves, however, are often another story. Once you've established your cooldown rotation for the tankbuster set, you can put your unused cooldowns to use in smoothing out the rest of the damage from these cleaves, or to troubleshoot areas where your healers are under more pressure. This allows you to stay out of stance for longer. Short recast cooldowns like Sheltron can be extremely powerful in this regard, once you've memorised the cleave timings. You can think of this as bridging together two low damage sections of the fight, bypassing the need to swap back into stance in-between.

[Reynhart]

Once you've established your cooldown rotation for the tankbuster set, you can put your unused cooldowns to use in smoothing out the rest of the damage from these cleaves, or to troubleshoot areas where your healers are under more pressure.

The cooldown rotation is set for tankbusters. In A1S, if you had 4 Plasma, PLD had to use HG for one of them, because it can't rotate on Rampart and Sentinel alone. WAR and DRK woudn't need it since they can use their CD sooner.

[Kabooa]

Lore reasons.

Didn't work for Haurchefant.