Originally Posted by
Recke
From what I've seen, at ~555 item level, a Protection Warrior can be hard hit/exp capped while also reaching (with proper gear) about 50% avoidance on the first table. The "why" is as others have explained. With the stats on gear as available (and how it scales), you can avoid every other auto attack from a boss (exaggeration, but referencing the 50%+ avoidance). Both Dodging and Parrying an attack will refresh Revenge, with Parrying providing a damage buff from Glyph of Hold the Line. And, with Riposte, you are converting 75% of both your Dodge and Parry ratings into Critical Strike, which is a significant dps gain, especially with Vengeance.
However, the reason for Stam gems should (might not be the reason, but it's a logical reason) be due to players believing they don't have the Health value to accommodate the current raid, which usually occurs when your ilvl matches or is close to the gear that drops on your difficulty. That being said, (example) if you are going into a raid that hits with a big AoE that does 400k to all players, and the boss does 150k auto attacks, you'd want to make sure your health value is high enough to live from a bad RNGed scenario (auto->AoE->auto in ~3-4 seconds). But the moment it is, and any time beyond that, you'd go towards other stats. More often than not, you'll see tanks, after they "outgear" an instance, aim more towards damage (example: Tanks using Stam Trinkets vs Damage trinkets or Tank trinkets that don't involve Stam).
That being said, between Dodge and Parry, Damage always helps. Having even a 1% better chance at avoiding something that won't kill you isn't really that big of a worry.