So 50 parry is 1% dmg reduction? I made BiS set focusing on det+crit and a set focusing on parry, the difference is a little over 100 parry. So thats 200hp on a 10k hp pool. 200/15=13 vit. That is the hp of a single i80 accessory if you would die from physical hits only. In return for wads of crit and det that might just force a phase change or kill faster. If that 2% on physical hits is make or break for you and the healers, you are probably severely undergeared for the content, and wont be having a BiS set anytime soon anyway.

I feel parry is mainly an aoe tank stat. Dropping 500 parry would be a bad idea, and is impossible, but up to 150 should not whipe raids.