Well, it was the day after and, scrolling through their posts I found an article on Retribution talents, gearing, rotation, etc. So, being the inquisitive and self-improving ret that I am, I took a gander.
To be fair, their normal paladin columnist was unavailable, so they had a priest columnist fill in.
But to be unfair, the columnist should never pick up a mace if he's going to actually walk into Icecrown with it and hit things. On the topic of talent builds, he said,
If you want access to Divine Sacrifice then you'd go 5/11/55 with those six points all in protection... I opted to go for the Divine Sacrifice build. It just seems to be something that I'll make a habit of using fairly often in raids. I'm extremely jumpy about defensive cooldowns.
Let's think about this: let's say a raiding retribution paladin has 25,000 health (some have more, some have less, this is just a nice round number to work with). With that build, the paladin can absorb as much as 50,000 damage (40% of the paladin's health multiplied by the number of party members = 10,000 x 5).
Note: Divine Shield no longer negates the incoming damage from Divine Sacrifice according to Elitist Jerks. Also, as the only foreseeable way to make this talent viable (Divine Guardian) is inaccessible to us, this analysis is assuming not taking Divine Guardian.
Did nobody tell Jimmy that bubble doesn't work anymore? Now he can't use wings for 30 seconds...
Ok, that's a decent chunk of change, but let's examine this realistically: say you're the ret paladin with the DivSac build in a 10m raid. You walk into a boss fight and DPS your little heart out until the raid leader says "DivSac, paladin!"
In order to be used as a tank cooldown, you must be in the group with the tanks. Duh, right? Ok, but in a 10m setting, there are two tanks and two other people in that group. Splash and AoE damage is all-too-common nowadays, so odds are you'll be soaking up some of your groupmates' AoE damage as well as tank damage. Since you have a set amount of damage you can absorb, the AoE damage your groupmates are receiving will take up a decent amount of that preset 50k damage and the effectiveness of that cooldown on the tank is basically naught.
Not only that, but in order to be COMPLETELY effective (i.e. soaking up the entirety of that 50k damage as tank damage), with no AoE damage whatsoever, the tank would have to take a hit of 166,666.66 (repeating) damage, which would be reduced by your Divine Sacrifice to 116,666.66.
To put this damage in perspective, let's compare that with our parse from our 10m Lich King kill. I'm using this because it's readily available and LK hits like a mofo. For instance, his melee swings hit me for an average of 12,726 damage each. However, I must take this number and factor in my armor's damage reduction:
Since my armor is sitting at 30,796, I have a total of 66.91% damage reduction, meaning that 12k hit I took was merely 33.09% of the original hit, which calculates to be 38,459 damage (rounding up from here on out).
(I can ignore dodge, parry, and block for this analysis since I'm looking at numbers from hits that I actually took.)
Let's do the same thing with that previous number. Say I was tanking the boss fight and a ret paladin popped DivSac in that "idealest of ideal" of situations. For me to take 166,667 damage pre-DivSac, the boss would have to had hit me for 503,678 damage. I'm not sure this is even possible for melee hits. Perhaps this could be spread out in a few hits over a period of time, but can't be longer than 10 seconds.
(Assuming perfect timing and a boss swing timer of 1 second, the 116,667 damage after DivSac would translate into 11,667 damage per second. Wow.)
Well, let's imagine we had a ret paladin popping DivSac during one of the Lich King's melee hits (not that it's a good idea ever, but just for the sake of argument). I took a 12k hit, so that means the paladin would absorb roughly 3.6k in damage. With tank health pools exceeding 60,000, and healers' spell power only going up, 3.6k is quite sad in terms of mitigation (ex: our resto druid's Rejuvenation was healing for an average of 3,246 per tick, according to the aforementioned parse).
In short: in order for DivSac to have any sort of usefulness as a tank cooldown (at least when untalented, as is the case in the linked build), the tank would have to take ridiculous amounts of damage with no AoE or other damage occurring to other members of the ret paladin's party.
"Ok," you may say, "that's kinda ridiculous. DivSac's true purpose is in mitigating AoE damage."
Let's look at that too, although this time I don't have specific examples (too lazy to dredge through our old parses and look for some good AoE numbers). Say that you, the ret paladin, are placed in a party with clothies and leather-wearers who have next to no means to mitigate AoE damage. Truly, this is the ideal situation for DivSac to "shine," right? Well, you're gleefully chipping away at Sindragosa when suddenly you're pulled in and she starts casting Blistering Cold. You all have your heads up your respective asses and don't run out. You pop DivSac and see what happens.
Every single party member gets hit for 30k. Your Divine Sacrifice can only absorb 50k, so ideally, it mitigates 10k per person. However, each person would still get hit with 20k damage. For most clothies and leather-wearers, this many people getting hit for a massive amount of their total health is unhealable.
(Please, please, please inform me if I am incorrect in assuming you get hit for the full 30,000 damage barring any Frost resistance and/or the tiny amount of armor clothies and leather-wearers have.)
All of this is assuming your healers are on top of their game and spam healing you so you don't trigger the 20% lower boundary. The millisecond you dip below 20% health, DivSac will cancel.
So let's see: with a 5/11/55 build (you could go 5/5 in Divinity if your tank sucks and you get hit a lot), you get a halfway-effective AoE damage-mitigation CD for your party only, whereas with a 11/5/55 build you get a 6 second period of double resistance to a certain school of magic for the entire raid (the most ideal use of AM), making it amazing for fights like Sindragosa and Blood Council (think when it jumps to Taladram and he casts a fireball). Just not worth it at all, in my not-so-humble opinion.
Edit: I've realized that I put most of the work (read: math) in this post to the part that doesn't really matter (using DivSac as a tank cooldown). This is the "damnit, I'm thinking about it wrong" aspect I mentioned in a previous post.