So in ffxi, if you had a shield it was helpful for casting. If you were casting and you blocked a hit it would prevent your cast getting interrupted, which was great for PLDs and WHMs alike. It was great that shield mechanics and casting mechanics went hand in hand.
In FFXIV they are completely opposed, you can't block or parry while casting. With moves like clemency and sheltron coming to PLD it would be nice if the blocking mechanic could be changed to be like in ffxi so PLD won't be hindered by casting and will instead benefit from blocking while casting.