Why Resilience Doesn't Suck (that much)
Crediting Oakfire, esteemed Angel Bobble, Xevious, and City Racer Champion, for making this connection.
If you are have Enrage...
While it reduces crits, Resilience does not reduce crit-activated abilities. Specifically, Enrage will activate.
However, the actual mechanics of the ability give it an added bonus. Even if you have reached the crit reduction cap with +Defense, Resilience will still proc Enrage.
Why is this? There's really nothing definitive, because two things could be occurring. Resilience may have priority to handle crits on the combat table over Defense, ensuring that all recorded crits get their intended Enrage.
Alternatively, there could be independent checks, and any percentage of effective Resilience is an equal percentage chance to proc Enrage. Given that Resilience only affects the target, not the attacker (for instance, a Warrior won't get Flurry procs if he hits Resilience on the combat table), and given that only certain spells are affected, I tend to believe that independent checks are more likely.
Either way, same result. :)
With Enrage, how does Resilience stack vs. Crit?
To word that a little better: if you have Enrage, is it better to get +crit or +resilience?
First off, there's no straight equivalence. We couldn't make an accurate one if we tried. Resilience procs Enrage based on how many times you get hit; Crit, obviously, is based on your own attacks. As a general rule, bosses will have a faster attack speed against you than you will against them, and any multi-mob scenario is going to move things in favor of resilience. Then again, Thunderclap slows them down.
For each percentage of Crit and Resilience:
Crit will give you 100% additional damage. With impale it will give 120% additional damage.
Resilience will give you 25% more damage on all attacks for 12 attacks or 12 seconds. This equates to up to 300% additional damage, and will allow heavy attacks such as Shield Slam to take the bonus.
Resilience costs considerably more in itemization points. There are, however, a small number of items that are tanking-oriented and also happen to have resilience on them. The Gladiator's Shield, for instance.
Talent Builds to utilize this?
There are two solid Prot talent builds which utilize Enrage and would make your resilience useful. Further, both of these pack an enormous amount of Threat potential.
Here's a tanking build:
And here's the uber Prot PVP build (didn't even need to change it!):
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Why did I skip Blood Craze on both of these when it is also activated by resilience? Cost effectiveness. If you do a lot of multi-target tanking, get Blood Craze instead of 3 points in Commanding Presence.
Fair Warning: Don't respec for Resilience unless you already have a good amount of it. There's no point.
Am I saying to stack Resilience if you have Enrage?
Nope. Not at all. Just like I have never advocated stacking crit on tanking gear (AP/Block Value are better!). It's only real tanking boon is if you have PVP gear but have not gotten a 490 Defense set (Defense and Resilience are additive, so you can reduce crit with a combination of the two).
However, I am suggesting that if you pick up a few solid items that happen to have resilience -- looking at you, Gladiator shield -- it's not a bad thing. :)