Might as well extrapolate usage here based on what FFLogs provides, as that's our only measure at the moment now that clears have happened. Note: This only reflects the current number of clears shown on logs at the time.

DRK: 22/25
GNB: 10/25
PLD: 7/25
WAR: 5/25

So, out of all that: DRK is highest in demand due to it being the top damaging tank, as well as having strong utilities in a raid setting that solidify it as a MUST HAVE apparently. The second spot is split between the three pretty closely, but GNB is more favored due to its damage being closer to DRK's even though three groups paired GNB with a PLD or WAR.

I think the current data tells us that DRK is too strong in terms of damage to be favored so highly - that, and both WAR and PLD are lacking in damage. You can say "Oh, but it's because of the utilities they bring!" but that's a poor argument because you'd rather be able to stack two magic mitigations compared to shields since it provides far more protection than any amount of shield can provide. It also doesn't help that Omega is skewed toward Magical Damage either, so it favors DRK/GNB far more than PLD/WAR.

Let's be real here though: If a single job's ability can't match another's in terms of damage mitigation, there's an issue with balancing as a whole. % Mitigation has always been favorable compared to HP Shields, as % Mitigations cut into the Final Damage far more. Shields are only as strong as their potency that subtract from the Final Damage before applying to HP. To equal in mitigation, Shields would have to match the exact % Mitigated by the percentage, and there-in lies the problem.

Not only do we have a 2-minute meta problem, we also have a fight design problem that needs to be looked at. If bringing shields(and a heal or regen attached to it) is weaker than bringing a proper party mitigation, then there needs to be a better look at how tanks apply their raid-wide mitigation aside from Reprisal and find a common ground where they're equal so one isn't burning more party mitigation resources than the other.