Quite frankly, if PD ever succeeds against a human player I think it says more about the attacking player than the defending nation
Seriously, a well scripted mage should be capable of burning through any reasonable amount of PD in short order, unless you're investing silly money into the PD. Even then, your relying on the chance of the mage either KO'ing himself with fatigue or simply lacking mass destruction spells to deal with the number of targets.
Oh, and sacred PD is only good if the AI happens to choose bless rather than holy avenger or similar.