I don't want to confuse anyone so...
1) Each additional % of dodge you have is better than the last. The extreme example would be going from 0% to 5% VS going from 95% to 100%. In the first, you dodge so little that your healers are still spamming everything they have as if you would never dodge. In the 2nd, you just might actually take two hits in a row once in a great while moving up to being invulnerable to physical attacks.
and 2) There are diminishing returns on dodge and agility. Where naked, the first 90agi (97AGI Talents) you equip might be worth (13.39%[180AGI] - 11.35% [83AGI]) 2.04%. And the last 90AGI would be worth (37.73%[1672AGI] - 36.47% [1575AGI]) 1.26%. For reference, this quick real number scenario was done as naked, talented Tauren upgrading to Greatness & then also DPS geared minus one trinket slot to adding greatness. The same 90AGI went from 2.04% in value to being worth 1.26% in dodge value. The higher you go, the lower the value you will get out of the next 90AGI.
So on one hand, each % of dodge is worth more than the last % because of *1*, but each point of agility, or dodge for that matter, is worth less than the last point as seen in *2*
This debuff Aura, is a flat 20% subtracted off what your dodge would look like before stepping into the instance. So in full IC gear, you might have let's say 56% dodge just standing there raid buffed without mongoose, idol, etc procs outside of the instance, but once you step inside, it would be worth 36%. At this high number of dodge (56%), you can be sure you're probably getting less per 90AGI than the 1.26% I have shown in dps gear by adding the greatness trinket. This DR effect will continue to gradually increase on both dodge and agility as you gear up.
I'm not going to sit here and pretend I can answer whether or not it will be a good idea to gem or otherwise focus on agility/dodge at those high of avoidance numbers. What I can say, and it's not saying much, is that the DR may or may not make it worth while initially and then once you get another few % it may or may not change what is more useful, but Stam suffers no penalty on any sort of DR.
In the end, some of it will likely come down to personal preference, and the rest might come down to knowing exactly what to expect on each boss because simple words hinting at tanks still possibly dieing to two hits initially getting into IC as GC has already suggested just isn't the same thing as actually being there or having someone tell you exactly how hard they were hit for.
I will leave you with what GC says on the matter.
GC wrote:
It arguably makes stam less important (though it will always be important for tanks). Many players are probably telling you right now that only stamina and armor are important because if you ever fail to avoid two boss hits in a row that you're going to die. Under that environment, avoidance loses a lot of value.
If bosses hit for less in IC (which they will, since they will hit more often (because you will have 20% less avoidance, not because the opponent swings faster)) then the value of avoidance for purposes of survival increases.
I still expect many tanks will die in two hits until they get geared up a little. But they will, and then the ability to survive two hits in a row won't be as big an issue.
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Avoidance relative value
If your conclusion is that anything that improves your avoidance is now bad as a result of this change, you should think through it a little more. If you didn't like avoidance before, nothing changes. If you liked avoidance before, nothing changes. You just have less of it now. The relative value should not change, unless you get to the point where bosses no longer two-shot tanks so much, in which case the relative value of avoidance increases.
The underlined is my added text based on my reading of one of the threads on the subject that GC has been posting in since it doesn't really come off as very clear. Many interpreted it as him saying bosses would swing more often which is not the case. The bold is my emphasis on his own text.