Don't confuse viable with optimal. Sure. It's not optimal, but it is completely and totally viable. There is only one optimal, and there will always be a preferred composition. There is literally no way to avoid this unless they make all the classes exactly the same, which from a gameplay and design point of view would be terrible. If they make PLD/DRK optimal then there will be posts asking if they will "make DRK/WAR viable" or "PLD/WAR viable".