There is zero reason PLD should exist the way it currently does. Cover is far too powerful of an ability in solo queue and mostly a liability in a light party setting.
In uncoordinated matches like in solo queue (thanks no chat), it makes working around Cover extremely difficult and oftentimes inefficient. There's no way to tell your party that we're just feinting to bait a Cover out and then switching to a second target once Cover is used, since the damage from 1 or 2 people once certain abilities have been blown is far too low to take down a target from full HP with no prep time (debuffs like Chaos Thrust, Sole Survivor and Butcher's Block).
One other option that a lot of players propose is stunning the PLD shortly before or during any burst windows to prevent them from covering in the first place, but that's STILL inefficient because it's forcing one stun to be used on the tank, freeing up the burst target or the healer. Just by Cover existing, it baits away CC from other party members with no actual input or skill from the PLD themselves.
If the Cover goes through, and you choose to continue, even with the 10% debuff on Cover it's still forcing players to burn through 14.5k HP or 15k HP EVERY TIME they want to get a kill. The PLD still has access to Hallowed Ground and very often Safeguard, even without Shield Oath. A Covering PLD almost always has way more EHP than any other target available.
I think it's perfectly fine that one of the tanks be considered the more defensive/support oriented option of the three, but Cover is far too oppressive and drastically changes the dynamic of a match. No other job in the game forces players to play so incredibly differently. Playing against different jobs does require players to adjust their playstyle a LITTLE BIT, but nowhere near to the extent that PLD forces people to play.
In a Light Party setting, it's not as big of an issue because players are in voice chat coordinating much better than you would in solo queue, so damage generally comes out faster and in much tighter windows, preventing the PLD from reacting our just outright killing them before they can do anything. It's for this reason WAR is generally favored over PLD in light party, when everyone is on the same job it's much easier to kill even through defensives or very easily baiting them out without wasting anything yourself. It would require way more work from the PLD's team just to get by.
I find that the biggest issue with Cover is just how often its up. It has a recast timer of THREE SECONDS, which is absolutely absurd. That means it's ALWAYS available, you have a very tight window where it's not available and your team has to be really on top of their game to get anything done within that window. Not only that but Cover's gauge cost is barely a hindrance, since every single global cooldown generates gauge at most they have to wait 7 seconds before they're allowed to toggle Cover on, which is still a much shorter time than The Blackest Night and Thrill of War.
Cover also has the luxury of being able to be toggled off before you use more gauge the the initial cost. It costs 25 gauge to use and 25 over time, but if you toggle it off the moment the option is available it saves you on 25 gauge. You almost never need Cover for extended periods, just by the nature of pvp burst damage is much stronger and more dangerous than sustained damage. As long as you split the damage with Cover by allowing one person to take a chunk of it then covering right after, you prevent a death on one while still preventing yourself from dying to any damage that sneaks through.
And yet for whatever reason, PLD still has access to the highest one GCD burst in the game? Royal Authority (or Rage of Halone, both the same potency) + Shield Bash + Requiescat is 3750 total potency in ONE global cooldown. That's higher than WAR's single GCD burst with Fell Cleave + Tomahawk + Onslaught which is
3250. Requiescat is basically Upheaval but with MP as a gate rather than HP. Even in a worst case scenario where a PLD has zero MP that's 2750 in one GCD, only a measly 500 potency less than WAR. How is that fair? Of course directly comparing things might seem unfair because a WAR has access to Butcher's Block in preparation for damage but that's an extra step a WAR player has to take to get full advantage of their kit, PLD has no debuff that they have to prime to deal maximum damage. And any debuffs that the enemy puts up are often wasted because Cover just redirects damage to the PLD player anyway, WITHOUT debuffs like Chaos Thrust and Butcher's Block transferring over.
Cover is basically all pros and very little cons, this ability is too powerful and it should be reworked or removed if they want the job to stay healthy. I've spoken with some high level players and potential fixes for Cover and we've come up with a couple of solutions:
1. Drastically increase Cover's cooldown but remove the gauge cost and buff other abilities.
By increasing Cover's cooldown, you make it so that the PLD player has to be much more aware and on top of their game when it comes to saving targets. If you increased it to say, 15 or 20 seconds, it's still just as available as The Blackest Night but still usable more often than Thrill of War, with no resource cost. They still have the strongest single GCD tank burst, their stuns are still available and you could potentially buff Clemency by decreasing the MP cost or increasing the potency, removing the MP cost and tying it's cost to gauge usage instead of MP.
2. Keep Cover's cooldown the same, but lock the PLD into Covering for much longer before they can toggle it off.
This option would harshly punish bad Covers by preventing the PLD from recasting the ability to save gauge and prevent them from Covering another target soon after. It would mean that Covers are much more of a commitment, while still giving them the flexibility of having the ability available often once it is toggled off. Most of the PLD kit could stay at current numbers if this were the case, and the 10% debuff could also be removed.
3. Remove Cover completely and replace with Intervention
This is the most drastic change, by replacing Cover with Intervention you leave PLD with a often available cooldown a la Cover but not as powerful which I think is fair if something can be used really often. You could have it mirror its pve counterpart, reducing damage by a % and then increasing that % when the PLD uses it while in Shield Oath. For actual numbers, I'm not sure what would be best. If it's 20% by default and 30% while in Shield Oath I think the gauge cost should be quite high, but if the numbers were lower, say 10%/20% it could keep the current gauge cost.