I crunched the numbers and found that stacking Parry is more useful for Paladin than it is for Warrior. This tiddle is for a Pally using a 110 or 115 Kite Shield. If your:

Parry is around 600(~30% parry rate total)
110/115 Shield(~30% block rate)

Here, your chance of mitigating a physical attack is a function of the probability of either one of these things independently occurring(with the small caveat that both cannot occur at once and Block is checked before Parry as far as we know). When they're both about equal, it peaks at about a 41-42% chance of one of them proccing. That is significantly higher than a Warrior can reach even in a max Parry build. I personally consider 50% as the threshold at which Parry rate will actually matter in terms of how often it procs, and that's actually pleasantly close.