Zaroua of Premonition made a really interesting post on the official forums discussing Holy Paladin Mastery today:
I'm creating this thread hoping to greatly reduce the influx of PMs and tells I get regarding our mastery. Keep in mind that is pretty much only for 25 man raids; I see some potential uses for this in 10 man, but for the most part it probably won't be as useful in a 10 man scenario.
The Firelands fights come in two categories: sustained AoE damage stages and no AoE damage/AoE damage that Holy Radiance isn't suited to heal. Beth'tilac, Lord Ryolith, Domo and the ground phase of Alysrazor are all mostly based around AoE healing for sustained amounts of time while while Alysrazor air phase, Shannox, Baleroc and Ragnaros are fights where Holy Radiance isn't even worth casting.
For the AoE fights, all we really have going for us is Holy Radiance and Light of Dawn, while all of the other healers are able to pump out AoE heals nearly non-stop. Now the way I look at it is that someone still needs to heal the tank and that since Paladins are pretty horrible at sustained AoE healing, a 25 man raid may as well put one or two Holy Paladins on full time tank healing and just have them Holy Radiance on cooldown.
For the non AoE healing fights, Paladins have the choice to choose between tank healing or... tank healing. Casting Holy Lights and Lights of Dawn on the raid simply doesn't compare to Chain Heal/Wild Growth/Circle of Healing/Prayer of Healing.
My guild's healing team is flexible enough to allow for a Paladin to have a weaker Holy Radiance in order to have more powerful tank healing. This translates in the AoE healers having to spend less time on the tank and more time doing what they're good at.
How does this relate to mastery? The golden rule of tanking and healing is that when it comes to handling damage, the best to worst order to do it in is this: completely avoiding the damage, mitigating the damage, healing the damage with a very large amount of fast heals, healing the damage with slow and large heals. Mastery helps mitigate damage and in some cases, completely avoid it. The reason why mitigating damage is so good is because it leads to reduced frequency of spikes in the tank's health and when spikes do occur, it makes them less pronounced, which in turn means that other healers don't panic and waste cooldowns or inefficient heals on the tank. Before the 4.2 change to make Mastery shield stack, the stat was mostly useless because such a huge portion of the shield was wasted on any given heal that gearing for Haste for faster reaction times was something pretty much every Holy Paladin agreed to being the better choice. But now we're in a position where Mastery is finally viable for a Paladin who wants to focus on tank healing.
The most important thing to note about healing with a Mastery set is that you sacrifice throughput in order to become a more effective tank healer. [Emphasis mine.] Your Holy Radiance (and mana regen) will be weaker than a Paladin who is going for a more balanced approach to gearing or a Paladin going for Haste. But in turn, you'll be putting a downright overpowered shield on the tank every time you heal him directly. And don't kid yourselves: if you're able to reduce the average hit the tank takes by 10k because of the Mastery shield, what you're doing is very nearly game breaking. The shield simply is that good for keeping tanks alive. What a full set of mastery comes down to is your guild's capacity to support one of its healers focus less on raid healing and more on tank healing.
Even if your guild can't (or won't) support a Paladin will a full mastery set, every Paladin should try to get off pieces with mastery on them so they can use them for Shannox, Baleroc and Ragnaros at the very least.
If you look at his gearset, he's gemming and reforging according to the following priorities:
Mastery > Intellect > Spirit > Haste > Crit
This has a lot of drawbacks. It's hyper-specialized for tank healing, and possibly even single-tank healing (no off-Beacon healing).
Now, you probably shouldn't run out and switch to this right away. But any time someone from a top Royalty guild like Premonition or Paragon says something that contradicts common wisdom, it's worth taking a good long look at the situation.