you do realize that the way vengeance and tanks currently work our dmg, no matter what setup or size, is important.
on a fight with no veng gimic....any progression based tank will easily reach numbers of around 100-150k for the 30secs burst period.....and maintain around 40-50k dps during the encounter. also during certain phases of heroic encounters (before its on farm) using a highly stacked veng tank will be more beneficial than most dps classes....for example a monk or pally tank on the adds on Feng will more than likely out dps half your dps.
so tank dmg is highly important and if used correctly (i.e ur tank knows how to gimic veng) can be the difference between a kill and a wipe.
not all classes have to hit expertise hard cap (if memory serves me correctly only pallys and monks have 2 reach it) however hit cap is the number one prio for any tank....then u decide the whole mitiagtion vs avoidance debate.
the old model of ignore hit and expertise and reforge full mitigation doesn't work anymore as tanks mitigation comes from landing abilities.
so its always (there are some fights where this doesnt count but 9/10 it does)
hit cap > expertise (hard or soft depending on your class and play-style) --> then ur primary stat. which differs from class - i.e dks go mastery, pallies go haste, druids crit etc