I'm also seeign a number of Tankadin with Seals of the Pure in their build. This is actually a bit more understandable since it buffs our primary seal, Seal of Vengeance (Corruption). But it might not be a great place to spend your talent points. It's another one of those 'it depends' types of answers.
So let's examine Seals of the Pure versus going deeper in Ret.
Well how deep in Ret are you already? In my humble opinoin, you should already be at least 12 points into Ret for Vindication. Vindication reduces the attack power of the Boss attacking you by 548 at Level 80. This is the exact same AP reduction given by Improved Demoralizing Shout, and Improved Demoralizing Roar.
I consider Vindication to be nearly as required as Judgments of the Just. It's a noticable decrease in the amount of damage you take. Just like JotJ, it can be provided by others in your raid, but due to target switching, mistakes ("oops I forgot to Shout") and player deaths ("Leetdpswarrior is dead."), it may not be available. The only way to make sure it'll up on the mob you are tanking is to provide it yourself.
For threat, the real question comes with Seals of the Pure versus Crusade. If you read Crusade carefully, you'll notice it double dips. It increases ALL damage caused by 3%. Then you get ANOTHER 3% bonus damage against Demons, Undead, Humaniods, and Elementals. For ease of writing, we'll refer to those as DUH mobs.
When fighting DUH mobs, Crusade will out threat Seals of the Pure. When fighting non DUH mobs, Seals of the Pure wins. (This was worked out in the 70 plus page Math thread on Maintankadin).
Look at what you are raiding. Heroics have an overwhelming majority of DUH mobs. Naxx is pretty much DUH from stem to stern. But in Ulduar you can't hardly find a DUH mob. Onyxnia's Lair is a DUH free zone. Crusader's Coliseum is all DUH except for Beasts. Most people expect IceCrown citadel to be filled with DUH.
Another talent I see many Tankadins taking is Divinity. I am of the belief that this talent was put in to encourage Holy Paladins to go down the Prot tree for their Bubble Spec (Divine Guardian and Divine Sacrifice). 5/5 Divinity is not a strong Tank talent and generally contributes only to more over heal.
Lastly, you can get by pretty good with just 1 point in Spiritual Attunement unless you are fighting General Vezzax.
So the Cliff Notes version is
- Spec 0/53/18
- YES to Vindication
- NO to Seals of the Pure
- NO to Divinity
- YES to Crusade
- YES to Seals of the Pure only if you spend the lion's share of your time in Ulduar
- YES to 1 point only in Spiritual Attunement