Stamina is mitigation, and it's "always on" mitigation. Ardent Defender/Commendation work at low health and the bigger that window is the less damage you'll take. I've had a skilled healer hold me at 30% health for long periods of time on purpose to give me insane mitigation (early heroic MgT Kael fights, 5k fireballs -> 3k fireballs, lawl) but that only works if you still have enough health to be reasonably safe at that low of health.
Like has been stated, you get stunned you lose all the benefits of your gems except for the miss rate from defense and stamina, same with fear or not having the mob in front of you. While in most fights you can control this and it isn't a concern, I don't see why you'd take the option that works only most of the time when there's a viable one that works all the time. Also, gemming for more avoidance gives you even more mana issues whenever you start to outgear a place, whereas when you're outgearing a place the healers have no mana issues, so who cares if they still need to heal you more?
The only places I've seen my healers go OOM is when the entire raid is taking massive amounts of damage, the damage a tank takes isn't enough to throw most good healers OOM. And most raid healers are spamming heals on you no matter what, dodging an attack just gives them overhealing, not mana conservation.
And finally, stamina is always useful. There are fights where avoidance means less, as you're going to take static amounts of unavoidable damage, as well as heavy magical attacks which defense/dodge has no effect on.
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