Divinity is pure over heals for a tank. Your getting topped up by spam heals 99% of the time, so why would you want an extra 5% healing when any good healer is already healing you to full and keeping you there? Now a holy pally would love this talent because it increases the healing they put out by 5% meaning they put out more heals for less mana cost. They also gain the benefit of 5% more healing on themselves if they get in a situation where they need a heal, especially since no one spam heals the healers. Next time you are tanking check the overheal meter and see alot of heals you received have become overheal anyway because you will have been spam healed during the fight, so divinity is adding to that with no real benefit, whereas you could increase your dps by a margin to reduce the fight length and increase your treat gen as a happy bonus side effect.

And, No, your build is not the most popular Tanking build. Mater of fact your build is about 4 patches out of date, and even when it was considered a "good build" it was a beginning tank build and by no means a raid build. 0/53/18 is the spec of choice for 90% of Raid tankadins. Its been shown time and again on EJ, tankspot, maintankadin.com, and the official paladin and tanking forums that Divinity is simply overhealing and not worth the 5 points. Seals of the Pure has also been shown to be far less threat generation then going further down the ret tree and picking up Vindication and Crusade. PoJ and SoC are both favorable to take as well as SoC ups your tps on trash and PoJ is a movement increase that helps substantially on several fights. Only 1/2 Spiritual Attunement is needed because you will be constantly spam healed thus regenerating mana, and you will have Divine Plea up 100% of the time if you have taken GbtL, so that extra point can be used in the ret tree as well.