Self-ascribed Parry advocate here: Parry is only useful for main tanks. There are some odd AoEs out there that can be parried, but when you're in that off-tank position, your role is typically to support and push as much damage as possible. I'd say it's only valuable in a fight if you're going to be tanking the boss or an add for at least 50% of the fight. This is why Syz' stat smorgasbord is generally a good idea while you're still getting gear: secondaries aren't going to make an iLvl upgrade not worth taking in a majority of instances, because you get more mitigation from Def/MDef/Vit than you ever could from Parry on gear, and you get more DPS from Vit/Str than you ever would from Crit/Det/SkS.

In an ideal situation, a tank would minmax their gear for every encounter, because each encounter has different requirements, and they'd refine a gearset that grants them optimum performance in each role (MT or OT) for each fight in progression. But that's a pipe dream, since gear is the carrot on the end of SE's stick, and since it is rightly time-consuming to obtain. At the very most, a tank might be able to accomplish a general MT set and a general OT set by the end of farming, but even that is a stretch.

If my math holds, and Parry contributes more to tank mitigation than their contribution to group DPS by the other stats, then I still think Parry is the best stat for a tank to take, unless they will spend substantial portions of time away from the boss' or adds' faces. But the adage someone once quoted holds true: a tank is, above all, adaptable.

I'll try to do some more number crunching this weekend to see if I can't find thresholds where Crit might outweigh Parry. Google Sheets shall be my friend even if it doesn't hold a candle to Excel.