I've been thinking about this question quite a lot recently. You now see items with bonus armour relatively regularly - neck items and rings in particular, but also my lovely new sword - the Crusader's Glory - and the trinket which can be acquired from the Emblem of Triumph vendor.
The short answer is, as usual, "it depends". Armour is subject to diminishing returns, because if it were not so subject then it would provide exponential increases in expected time to live (ETTL) in return for linear increases in gear. My understanding of the armour DR calculation is that it smoothes out the ETTL curve into a straight line, and therefore it should be possible to say that 1 point of armour value increases ETTL by a fixed quantity. Rather than get heavily into the maths in this post, however, I thought that I would put a few examples down to try to illustrate how we should all be thinking about armour.
So what follows is an illustration of the type of benefits that armour can bring. It's not a "choose armour over stamina in this ratio" post.
Suppose your current armour levels are sufficient to provide 63% mitigation against physical damage. You then add a ring which has bonus armour and increases your mitigation to 63.5% (at my gear level this might be 600 armour or so). You have gone from taking 37% damage to 36.5% damage from each hit. Your physical mitigation has therefore increased by 0.5/37 = 1.35%.
Suppose that before you added this ring a particular boss used to hit you for 15k per swing. It will now hit you for 203 damage less per swing.
Suppose that before you added this ring a particular boss used to hit you for 12k per swing. It will now hit you for 162 damage less per swing.
Suppose that before you added this ring a particular boss used to hit you for 18k per swing. It will now hit you for 246 damage less per swing.
So what do these numbers tell us?
The most important point to note is that the absolute benefits of armour when compared to stamina/blocking increase the harder a boss hits. The same is true for avoidance. If you are mainly tanking heroics and Naxx, armour may not be your best stat to stack. You may prefer stam/block. If you are tanking hard modes, then armour becomes more important.
The usual caveats apply too - armour does not help you to survive magic attacks or bleeds. In the game at present magic attacks (plasma blast, frozen blows) and bleeds (Gormok's impale) are some of the most deadly single attacks. It's also worth noting though that Ferocious Butt and Surge of Darkness are large physical hits.
It is a long-established argument that in the current content it is a single hits and spike damage that wipe a raid, rather than sustained tank dps. With the level of tank damage incoming in some of the more recent encounters I'm not sure that's true any more. Certainly it is worth running some numbers like this when deciding on a new piece of gear.
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