There is no amount of parry that will let me say "I can parry this Death Sentence (or whatever other huge hit move) and not all these mostly meaningless auto-attacks." That's the issue. It will always be inconsistent because it is an RNG stat. It is a stat that won't even come into play 100% of the time because of how it works with positioning. It is not possible to will yourself into parrying any one specific hit, while you can definitely pop a CD for a specific hit. If parry were able to be pushed high enough to be that effective, then it would be exceedingly obvious wouldn't it?

I think S-E learned from XI that mitigation based tanking is hell. No one likes being wholly dependent on a lucky die roll to save their ass.