A writer on EJ did enough tests to confirm the length of [spell]bone shield[/spell] internal cooldown, for each charge to be used up: 2 seconds. It might be slightly lower than that, in fact, but its certainly no longer. He got 40 mobs to attack him and looked at the time between bone shield activation and it fading.
Bone shield is not 2.5 or 3 seconds, as often quoted around the internet. It’s a measly 2 seconds, maybe slightly less. Since the first charge is used up at time zero, the last is used up 3 charges later, at six seconds.
So in Icecrown with fast boss attack speeds versus four bone shield charges when glyphed, we believe the least uptime you can get is 6 seconds. You could get more uptime if your avoidance RNG is in your favour. That’s horribly bad compared to [spell]unbreakable armor[/spell] which is always 20 seconds.
I’m not counting uptime when you are out of combat, of course, not counting time when you’re not taking damage that uses charges.
Bone shield does scale with avoidance, but you have a lot less of it Icecrown, and bosses hit faster.
I would not main tank with Unholy in Icecrown raids.
Merit? Where bone shield appears to have merit is whilst kiting oozes on Rotface, since its uptime is great when you’re not being hit. It doesn’t help as soak tank on Blood Queen since (I believe this is right), none of that damage can be mitigated (ie. stamina is your solution). It suits any fight where you’re not actually the main target or where the boss hits slowly or have a role where you run around without bone shield charges being used up (I don’t think any of the Icecrown raids are in that category). Looking over the other fights in ICC, I don’t see any jumping out as having mechanics that suit bone shield or AMZ either.
It’s one remaining selling-point is that if you do not have an Unholy DPS DK, it might be worth the tank going Unholy just to spread the debuff, if your gear is good enough to compensate for not really having a useful cooldown.
The two-piece bonus [spell]70650[/spell] does not suddenly make Unholy tank spec particularly shining, instead it’s more useful to think of it as fixing Bloods’ crappy AOE. Whilst you could conceivably do a D&D-based single-target rotation for quite high threat, it doesn’t suit mobility fights and it’d pull agro on Saurfang’s adds, and your bone shield is still crap.
Unless someone comes up with a list of surprise uses for unholy tanking, I think Unholy is pretty much dead for use in Icecrown raids. I wouldn’t even use it as an offspec for AOE; would go with Frost instead since UA is much better to mitigate rushed incoming damage from AOE tanking.
(that grey stuff was my caveat )