Regarding the use of defense as a mitigation stat, and the point at which it's more useful than agi or dodge rating:
First: You have DR on each avoidance stat. Dodge, Parry, (Block?). It's unknown whether Miss has DR, last I heard.
Second: DR is separate for each avoidance stat.
Third: DR is based on stats, not talents, so 10% dodge from bear isn't factored into it.
Now, what was mentioned earlier, about defense passing dodge/agi as a pure avoidance stat is based on the idea that the miss from defense gains more value as the dodge from dodge/agi diminish.
IE, suppose you have 400 (arbitrary choice) budget to spend, defense, dodge, or agi.
39.35 dodge rating per 1% dodge at 80
~41.67 agi per 1% dodge as a bear
~4.92 defense rating per 1 defense, which converts to 0.04% dodge and 0.04% miss.
So, we'll compare dodge and defense, though agi and dodge rating are quite close anyways.
400 budget as dodge rating will give us approx 10.165% dodge, ignoring diminishing returns.
400 budget as defense will be approx 81.3 defense, which gets us 3.252% dodge and miss, total of 6.504% avoidance.
Obviously, dodge gets us the most in this situation.
Now, using the formulas here:
http://elitistjerks.com/f31/t29453-c...gs_level_80_a/
(This is really rough napkin math, just trying to illuminate the idea fully)
If we total just 10% dodge, that dodge rating is worth around 9.5% dodge.
If we were at 10% when we equipped this 400 budget worth, we'd go from 9.5% to around 17.5%, IE an 8% gain.
If we were at 20%, we'd go from 17.5% to 24.42%, a gain of 6.92%.
30% worth of dodge, we'd end up with 30.4% after DR and the additional 10%ish. A gain of under 6%
So, let's say we're earning 0.6% dodge-per-1% of dodge-from-stats, which should occur around 40% from gear.
This also affects the dodge gained from defense, but it does not affect the miss chance.
So, 10% dodge becomes 6% dodge, but the defense we get 3.252% miss and 1.9512% dodge, 5.2032%.
In short, most people aren't going to be butting up against these right now, but I'd bet during ulduar the numbers start to turn in favor of defense.
Also, I may have mis-applied these a bit, so if you're really curious, get the function and plot it in some graphing software. There is a point where dodge rating/agi are worse-per-budget than avoidance, and if you start considering getting both on the same item, people can certainly already get more avoidance from that (not everyone, but some).
Hope that helps clear things up!