Mathematically,
ages ago, it's been shown that crit/haste reduce more damage over a long period of time than mastery, which I assert is worth diddly squat. Tanks don't die during long periods of time due to healers running oom, tanks die to spikes, such as this following example:
I was tanking 3 windblades on heroic Empress last night with my (crit gemmed/reforged exp capped) 496 brewmaster alt.
The result? When my avoidance failed, I died:
http://www.worldoflogs.com/reports/r...=13629&e=13977
Shuffle was up, stagger was only ticking for a mere 17k right before the burst, and all of my cds had been exhausted by that point. Yet I still splatted inside 3 seconds with random spot heals. EH is still a decently important consideration.
If I had say, 5% more mastery, then I would most certainly have survived that section, and we would've killed empress yesterday instead of having to go back and reclear trash today. But since I mainly tanked normal and challenge modes on that character, I was not geared correctly for such a fight and caused at least one wipe.
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While my enchant comparison was a little off, it certainly still carries weight. At the instant of death (and for the preceding five seconds or so), crit and haste are irrelevant. You either didn't have the elusive brew buff up or you got hit through the avoidance, doesn't matter. Shuffle is always up and a significant stagger is almost always purified (in above example, stagger was ticking for 17k until the final second before death where it jumped to 50k). The only thing that will save you at that point is stamina, armor, and mastery, of which armor we have basically no control over.
So yes, in life-or-death scenarios, having a significant amount of mastery helps, and it takes from stats that are irrelevant to your death.