Well... The major difference between PLD and not PLD is that while PLD has a shield to block attacks to provide that extra layer of mitigation, DRK and WAR instead have lower CD's on their mitigation CD's allowing them to use Vengeance/Shadow Wall more often than a PLD can use Sentinel.

To say nothing of being able to use Dark Mind and Thrill of Battle which again have shorter CD's than Bulwark.

So, it's expected that PLD would have to use fewer CD's because that's how they're designed. More passive mitigation but longer CD's so they can't pop skills as frequently.

Leveling up and doing Roulettes, I don't honestly feel much difference between the tanks in terms of defence. They all feel similar, given I manage their CD's properly. That is, until level 52 PLD because then Sheltron is really, really good against bosses. Since it lets you go ham in DPS stance while still letting you block 20-30% (Depending on what ilevel sync is putting your stats at) of the major damage abilities of bosses. Which is never matched by WAR (Unless you sit in Tank stance for Inner Beasts...) and DRK only compares at level 70 with TBN.

Though, this only matters if you don't need to be in Tank stance to keep aggro because some DPS is going ham and doesn't know what Diversion does... Since if you're in Tank stance, not only do you basically not generate gauge ever (Until level 68 when you'll have Requiescat > Holy Spirit spam which generates gauge in Shield Oath) but also you then also have higher base mitigations for the tanks across the board (With Grit also giving damage reduction and self healing from Souleater and WAR having Inner Beast) so the difference between having that block and not isn't as large.