PLD can Block without Sheltron already. It's about 6% or 9% stronger than parry if I'm not mistaken. It just depends on what kind of shield the PLD has that judges how often they block attacks. Depending on the strength of the shield the block rate is less.
I think the Shield types are:
Buckler - High Block Rate / Low Block Strength
Kite Shield - Medium Block Rate / Medium Block Strength
Tower Shield - Low Block Rate / High Block Strength
