The PDT sword comes in handy mainly for holding monsters/pets in Legion/VW, specifically monsters where you do not have capped blocking rate with Ochain. Since you're not damaging these monsters you don't really gain much by equipping a DD sword and the extra PDT enables you to use +Shield Skill in your other slots and helps to prevent those risky "Spell Interruptions" when Palisade is down . Of course if you have a good healer you could do without, but that is not always the case.