Maybe I am missing something but I feel like the way the game works right now, the designers intended Mastery to be a super great stat for Holy Pallies. However, as it’s been, stacking haste seems much more effective (maybe I am wrong on this). As I am sure you know, the next patch is (slightly?) increasing the effect of mastery for holy paladins. In your opinion is this going to mean we should start mastery instead of haste, or was mastery never something the game intended us to persue?
Even with the changes coming in 4.1, most theorycrafters seem to agree that the holy paladin mastery (Illuminated Healing) will not be as good as haste. Mastery rating might end up slightly better than crit rating, but will probably be more or less at the same level, and still below haste rating (and definitely below Spirit).
The problem with our mastery is two-fold. First, Protector of the Innocent, Holy Radiance, and Beacon of Light heals do not trigger it. And those three abilities comprise a significant portion of our total healing.
The second problem is that our mastery does not stack. Let's say you cast a heal on a target with a mastery shield already on it. If the new shield is larger than the remaining shield, the new shield replaces the old one. If the new shield is smaller than the old shield, the duration of the old shield is refreshed. Either way, you end up with less absorption than you would expect.
In my experience, Illuminated Healing accounts for a bit less than half of the number stated in the description. That is, if the description states that the shield is 10% of your healing, you can expect it to absorb 4-5%.
Now, I don't think Blizzard is entirely happy with our Mastery. I think they like the general idea of Illuminated Healing. To be honest, I like the general idea too. In theory, it's a nice complement to our direct healing style, providing a small buffer for the next bit of damage, and as a nod to our previous history of tank healing.
What I believe is happening is that Blizzard is trying to avoid an exploit with our Mastery. Let's say that the shields stacked. The obvious exploit is to cast a ton of heals on the tanks before the fight starts, or during lulls in the fight, and build up a giant shield. We would be spamming heals on the tank even if she was at full health, and that's not a healthy or desired play-style.
Ideally, shields would "roll" like Ignite does. The shields would stack, but 8 seconds after a shield is cast, whatever remains of that particular shield disappears. So the shield can't hang around forever, but they don't get wasted before their lifespan expires. However, there might be technical issues with this. I remember it took a long time for Ignite to roll correctly. Even recently Blizzard removed Ignite triggering on periodic spell ticks in order to keep it working properly. And Illuminated Healing is actually more complicated than Ignite, because the shield is reduced by damage in an unpredictable manner.
There are other options, including allowing the shields to stack but having a maximum shield size (maybe dependent on how much Mastery one has). But that still gets into the whole pre-healing in order to get a max shield up scenario.
Another alternative is having the shield only count on actual healing done, not over-healing. But we all remember how dicey that was when Beacon worked that way. It might work out better now, with the larger health bars, but it might not.
Or maybe have the shield stack, but lose 20% of its current value every second. Actually, this probably a bad idea, I think the math on it looks weird, and it's still vulnerable to pre-healing.
So as you can see, there's really no "nice" solution for our Mastery. Blizzard's current solution is as good--or as bad--as anything I can think of. In the end, eventually raising the value of mastery will get it to a point where we will use it.