Regardless of how good or bad parry is, what else would you gear for? Speed requires more than you can realistically get to make a noticeable change. Crit is nicer on WAR to be sure but for PLD, base crit rate is so crap that you're gonna need a lot to see a big impact. Determination is good but takes a lot to add a little. Parry is 1) readily available on most tank gear and 2) itemized to a large amount of the accuracy pieces. Whether it is a meaningful form of mitigation isn't important, if your raid needs more damage, it shouldn't have to rely on the tanks to carry the dps check.