Having a 100% magic dmg boss like A4 does make DRK look like a good choice compared to PLD. If the Alex 4 Savage raid was this exact fight but with much higher damage going out, I can see DRK coming in in place of PLD, but PLD has some nice tools despite not having any physical damage going out from the boss. Shelton/Bulwark is very powerful against the quarantine add, especially when you don't have a healer to help you out. Stance dancing to Shield Oath to soak orbs feels a lot better than swapping into Defiance (since Defiance increases your max HP but awkwardly doesn't heal you for that difference). Not having to use Rage of Halone improves PLD OT dps by a lot. Divine Veil is useful against the megaflare-like move (forget the name). No matter the fight, Hallowed Ground is always good.

It's funny how an encounter needs to be designed to completely negate one of PLD's main advantages (shield) to consider bringing in DRK as an optimal pick.