They arnt apples to apples. But a tank is largely healing themselves after tank busters (which are circa every minute) or when the healer is somehow tardy. In that light OGCD burst healing is better and the warrior kit is great for it. Paladins have an advantage that they could spend their mana on on more healing, which would be similar to a warrior giving up fell cleaves for healing in a way. One is not strictly better than the other, they are different. OGCD is better however in the sense that you don't have a healer dc/dead.
Switching to Defiance is not a GCD. You can do it while in the animation lock of other abilities. More over, casting clemency has the same cost for paladin, giving up their damage output for Holy Spirit as well as their gauge regeneration which is needed for their MP restore. Fell Cleave can also be 'dumped' before moving to Defiance for any kind of planned healing which is the standard in FFXIV fights (i.e you know the tank buster is coming) making it less of an issue.Which is a GCD, and forces to rebuild a very valuable ressource for your damage output. Because without Beast, WAR lose much more than PLD without MP since the WS potency are closer to Holy Spirit than WAR's to Fell Cleave, especially since PLD could stay in Sword Oath.
I never said the bonus is not better used, only that if you are forced to make an emergency decision then there is a benefit to it. Its not as if you are out your berserk while in defensive stance either. You are still enjoying bonus damage over normal, just not ideal damage of course. Secondly, Requiscats bonus healing only occurs when the ability is used at over 80% mana, and most of a paladins rotation is set up to blow their mana on holy spirit and then build mana back up before the minute cd of requiscat is back up. Given requiscat is used in the opening rotation, the world in which you are back up to 80% mana with requiscat off cd and decide 'oh i need to use this to heal myself 5 times' is utterly silly.Sure, you have one heal skill that gets a 30% bonus, but that bonus is better used on something else. While Requiescat can apply a 20% bonus for up to five Clemencies without any penalty.
On top of that, those 5 spells can also be boosted by Convalescence, while WAR's can't profit of the whole duration.
Could a paladin do it though, spend all of their mana, one of their offensive cds and spend 12.5 seconds(recsts)/5 gcds to be a heal bot for a little while? Yes they can. RDM can do it as well through veracure spam if they are so inclined. But we are talking about what a class can do through very inefficient gameplay centered around the idea that one of your party members has DC'ed. The paladins damage is effectively auto attacks for 12.5 seconds to pull off that trick and their rotation is hampered. Warriors have less 'potential' self healing, but the self healing they have is easier to access and does not interfere with their ability to hit buttons, and given the very predictable nature of fights in FFXIV, its silly to not value how good that is. Every time a buster comes up a warrior can pop inner beast + either rampart or vengeance, then equilibrium the moment its struck before moving back to offense mode after eating up their meter.