That is why I listed durations in the examples I wrote. For example, I wrote that Holy Sheltron's barrier could be 12s duration. Holy Sheltron takes about 22s to generate (Oath Gauge fill rate), and so while you have it up more often than not, you're still going to have ~10s out of every 22 where you don't have the barrier active. You can cover that gap with a different barrier, EG from Guardian, or Divine Veil, or maybe Divine Might/Confiteor/Blade combo barriers are up at that time. But eventually there will be points in the rotation where you don't have any barrier left to put up, and the damage comes in. And once it does, you can't recover that HP without a Healer's Healing, unless you use Clemency (which you will never want to do, it's a DPS loss). That's the point: There's going to be gaps in your defence, and it'd be an aspect of 'player skill' to rearrange when those gaps are presented to the enemy, so that they are timed to line up with the weakest moments in the enemy's assault.
Also, how much do you think these barriers are protecting you from? A 1000p heal in my last-tier gear on PLD (not even BIS) is like... 38k. So, a 400p barrier is 15200 HP. But, I mentioned that these PLD barriers would be 'stacks of barriers' in this redesign, akin to how Haima/Panhaima works. So, if an attack deals more than 15200p, the second layer of barrier doesn't catch it. It goes through and hits real HP, before the next layer is primed to take the next hit. For reference, this is the first double-pull in the latest dungeon:
This is one autoattack per enemy in the pull (there's 5 of them), and deals a fair bit more than 15200 damage. But, not pictured, is my buff bar. I have Shadowed Vigil (40% mit) and Rampart (20% mit) active in this picture. So, using two of the stronger options, together, is still not enough to take the pull's danger factor down to 'the enemies break less than one layer of barrier, per round of autoattacks they throw out'. In fact, we can see I parried one, and how much it dealt as a result. I could parry every incoming hit, along with having SV and Rampart active, and it'd STILL eat through a 400p barrier in one set of autoattacks (with 5000 damage left over to hit my 'real HP').
If we go into an EX roulette as SGE, and use nothing on the tank but Panhaima (6 layers of 200p barrier, 1200p total), you can also see how fast the layers get eaten through in a different format. They last 15s before falling off naturally, but they don't even make it to that long. If we then assume that PLD's Holy Sheltron (4 stacks of 250p, totalling 1000p, a little less than Panhaima totals up to) functions similarly, then we can estimate that it'd be eaten through about 20% faster (given it's around 20% less total potency). In seconds terms, if Panhaima takes 6s to eat through, HS would take 5.
What also has to be considered is 'timing'. If you have a barrier active, but no damage comes to use it, then the barrier is zero-sum, it didn't help you. If you use self-healing, the only way it can be zero-sum is if you're already at full HP. As a tank, the chances of being at full HP are pretty few and far between. You are almost always going to be able to 'make use' of the healing, unless you're the OffTank, in which case SE has handily made it so you can give your unrequired selfhealing to your CoTank for whatever reason. With Barriers, however, you cannot just hit the button willynilly. If you press Equilibrium at 50% HP currently, you get HP and it's 'useful' (because it restored HP). If you press TBN currently, but take no damage for 10s after pressing it, the TBN falls off and it was 'useless' because you spent MP on a mitigation that didn't mitigate anything. The same logic applies across the whole idea: Holy Sheltron's healing ALWAYS comes in useful, because if you press HS to mitigate something, it's because you're taking damage from it. And once that damage is dealt, the HOT of HS starts healing your HP back.
Yes, because even though it has less self-healing potential than the other tanks, DRK still has some: Abyssal Drain. I was going to write this in the post you responded to, but I decided against it becuase I couldn't work out how to word it. But, since the conversation went in the direction I hoped it wouldn't end up going in, I'll just say what I was going to say originally.
Do a run of EX roulette, with 1 DRK (you) and 3DPS. Unbind Living Dead and Abyssal Drain from your hotbar entirely, as they both have self-healing components to them. Because it'd mess with your rotation too much, I'll say that Souleater is still allowed (despite it being the strongest self-healing power of all the Tanks' 123 combos). There's not much that can be done about Shadowed Vigil healing you for 1200p, but if you want to, you can manually make note of your HP level at any given moment, and subtract the amount SV heals you from it. If you dip below the SV heal value at any point after using it (I estimate it'd heal about 50k), you can consider yourself 'dead'
My theory is, you'll get through the pulls, yes, but it'll be far tougher to do so without the 'instantly hit full HP again' effect Abyssal Drain is providing you, the 1200p Excog of SV, and the weird healing effect of Living Dead.