As it's already been mentioned, Parry has one of the worst scale rates out of every stat in the game. Getting a notable increase with it is like pulling teeth. It was actually mapped by someone back during the Zeta grinds (when tanks were debating which stats to put on their original Novus weapons) and it was proven not to be worth it. I was gonna post the numbers here, but I can't seem t find the original post, and I'm too lazy to search for it. The only tank who can create a noticeable increase in their parry rate are Dark Knights using Dark Dance, but that's because the CD scales based off of a Percentage, not a stat increase. The stat itself is terrible, which means that the Parry defense is, basically, pure RNG. If it happens, it's great, but it's definitely not something to count on or build as the baseline of your defense.

Instead, it's much better to look for DET as a secondary stat. The reason here is not just for Dps output (killing quickly is great, but we have Dps for that), though that plays a part of it. The way in which Det scales and effects your Str stat, as well as the increase in recovery magic, provides far more mitigation to a Tank that parry. For one, it's not random. The results are immediately noticeable. stack any tank (including pld) with Str accessories instead of Vit accessories and you'll notice a difference in output. Increased output = increased enmity, increased hp recovery from moves such as Bloodbath or Inner Beast, and increased healing magic potency (such as clemency, in the case of Pld's). These are all things you can bank on by planning your CD's appropriately, unlike Parry which is almost completely random (Drk's aside).

TL;DR The bottom line is that a Tank's defense isn't truly based on secondary stat allocation so much as it is based on knowing your defensive cooldowns, which CD's work together and have synergy (ie: Pld's popping Fight or Flight with Bloodbath is a MUST. Popping Sword Oath as well grants even better returns) , and how you can best exploit them. When you know these things, you plan your secondary stats accordingly so that you can greater maximize the effects of your CD's. Parry NEVER acts as an optimization for any Tanks CD's (including DRK). DET and Crit, however, can greatly increase the effects of a tank's CD's when they are used correctly.