If you assume that at present the standard raid tanking spec includes deep wounds, then there are only a few spare points to throw around. I personally consider improved disciplines to be baseline too, which means that one of the key decisions that tanks make when speccing is whether or not they want to max out shield specialisation. Two points in this talent are required, and it often comes down to whether you want to add a further three talent points, or whether you want to go for three points in focussed rage instead.
In my opinion shield specialisation is unquestionably better for progression fights, whereas focussed rage might be superior for heroics and farm content.
My reasoning is that firstly shield specialisation provides mitigation. Tanking is all about taking less damage, right? Taking 1600 block value, and bearing in mind the 60% chance for critical block, you have roughly a 1% chance per swing to reduce melee damage by 1600 and a 2% chance per swing to reduce melee damage by 3200. You can refine this further by excluding avoided attacks and attacks which are already blocked - suppose dodge chance of 6%, parry of 21%, block of 16% and miss of 10%. This means that, assuming the boss swing connects, you have about a 2% chance to reduce boss damage by 1600 and a 4% chance to reduce boss damage by 3200.
Tank gearing is all about taking lots of small numbers then adding these together to make big numbers - lots of small mitigation abilities combine together to provide a significant degree of protection. I think the extra block chance provides meaningful extra mitigation.
In order for focussed rage to be better than maxing out shield specialisation, it needs to provide a good enough rage boost such that we are happy to forego the extra mitigation. So let's look at the benefits of focussed rage.
The first point to note is that rage is capped at 100. Suppose I have 100 rage. I use 3 abilities, reducing my rage to 40. The boss then hits me so hard that it would be worth 80 rage to me, but since I have 40 rage my rage gain is capped at 60. Note that focussed rage has been precisely zero benefit to me in that period. If I had specced into focussed rage I would have had 49 rage after having used my three abilities, so my rage gain on being hit by the boss would have been capped at 51 instead. I would have had 9 extra rage as a result of using focussed rage, but this extra rage would have been wasted as soon as the boss hit me.
"Aha!" you might say, "but what if you avoid the boss's attack?" Well yes, if I were to avoid a long string of the boss's attacks there is the possibility that I might save enough rage from focussed rage to get an extra attack when I would otherwise have been rage starved. Bear in mind however that avoided attacks also grant you rage under the shield specialisation talent.
For the purposes of this example let's assume that the boss hits you every 1.6 seconds (1.8 is the norm in ICC I think), you have a 1.6 second swing speed on your weapon, and your reaction times are not great so you press a GCD-limited attack such as devastate every 1.6 seconds. Thus you have 2 attacks - heroic strike and a GCD attack - for every incoming boss attack. Focussed rage saves you 6 rage between the boss's swings, whereas if you avoid the boss's attack you gain 3 extra rage from shield specialisation in that time. Therefore if you have a string of say 3 avoided attacks, which is pretty rare these days, taking focussed rage would have gained you just 9 extra rage over maxing shield specialisation - not even enough for a devastate. That's 9 rage over just under 5 seconds too, and if you hit revenge (5 rage), shield slam (20 rage) and devastate (15 rage) in that time, plus three heroic strikes (36 rage), you would still have rage left anyway.
So, in short, focussed rage is pretty useless when you are regularly getting whacked over 100 rage by a boss. Where it comes into its own however is when you are not getting whacked by the boss - when the boss is casting and when you are the OT in a tank swap fight. One of the most annoying things about the LK fight is running out of rage when you avoid the hit before he starts channeling something, and then you have four seconds of him casting before you start getting rage again. When the boss is not hitting you shield specialisation gives you nothing at all, and focussed rage shines.
So there you have it - shield specialisation gives a mitigation benefit that is not trivial in my view. When you are getting hit by a raid boss there is not much difference between the two. When for whatever reason the boss is not hitting you then focussed rage is a significant dps boost.
Your choice is whether you want the mitigation, or some extra dps when not getting hit. I have chosen the former.
EDIT - Corrected my napkin maths!
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