Get past your bias
I will freely admit that haste is my favorite secondary stat. I have always liked the idea of lowering the global cooldown, and I hope to eventually get to the five-tick Holy Radiance haste break point. While haste is definitely awesome, it has one major flaw: It doesn't increase the HPM of most of our healing spells. I love haste for the HPS it brings, but casting more spells means using more mana. Critical strike rating and mastery rating don't have that penalty.
If you look at most of the holy paladins breaking records on World of Logs, they're almost all reforging into haste rating. AoE healing is the only way to generate enough HPS to land on top of WoL, and so you're going to see the AoE-focused holy paladins doing well. The truth is that having high HPS numbers don't always equate to being the most effective healer, especially when considering that many of the top holy paladins don't even make their parses publicly available.
There's nothing wrong with stacking haste rating on your holy paladin, and in fact, you'll be in good company if you do. My point is that the other secondary stats have come a long way since Cataclysm's release, and they're strong enough to warrant a second look.
Mastery has a bad reputation
Illuminated Healing was very bad for a very long time, and so it's understandable that holy paladins are still apprehensive about picking it up. The truth is that it's been performing pretty well as a stat since Firelands, where several holy paladins switched from haste to mastery builds and saw success. Mastery provides significant and fairly predictable HPS and HPM increases and boosts a raid's survivability by raising a player's effective health.
The problem with mastery is still finding a situation where it's always going to be useful. What I do is figure out where discipline priests are thriving, because that's where Illuminated Healing will thrive too. Fights like Yor'sahj and Ultraxion require tons of healing, and the sheer amount of AoE raid damage being dealt ensures that our bubbles will be fully utilized. Holy paladins facing heroic Yor'sahj favor mastery builds to counter the purple ooze debuff, while heroic Ultraxion makes haste irrelevant by giving us the Source of Magic buff.
Mastery is a great stat if we know that our raiders are going to take damage soon, and Dragon Soul has enough AoE damage on every fight (save Hagara) to make mastery an attractive option. I have seen several holy paladin parses where Illuminated Healing is near the top of their healing done, simply because we get a lot of potency per point of mastery rating. Our mastery won't be valuable on every fight, but it seems to fit in with the Dragon Soul encounters.
Critical strike is valuable, but unpredictable
Critical strike is always one of the tough stats to figure out as a healer. Doubling our heals for free sounds great, but the catch is that we can't control which heals are doubled. The fact that neither mastery or crit is completely reliable or relevant in all situations is a major reason in why they've been considered to be weaker than haste. Critical strike rating does increase our HPS and HPM by more than both mastery and haste, as long as all of the critical heal was effective healing.
The Spine of Deathwing encounter, with the Searing Plasma debuff, provides the perfect conditions for crit rating to shine. Imagine that every raider has 400,000 health when they land on Deathwing's back and that when Searing Flame gets applied, they lose 200,000 life. We have to heal that health back up quickly, and since there's such a large health deficit, any critical heals we get will be completely effective. There are many holy paladins stacking more crit on their gear to survive the heroic Spine of Deathwing, as the artificially inflated health pools are perfect for ensuring that critical heals aren't wasted.
I still value crit the least of the three secondary stats, but the takeaway is to realize that it's not always pointless. Critical strike rating can provide us with a massive HPS and HPM increase in the right situation, and knowing that we have that option available to us makes us stronger healers overall.
The Light and How to Swing It: Holy helps holy paladins become the powerful healers we're destined to be. Find out just how masterful mastery healing can be, gear up with the latest gear, and learn how to PvP as a holy paladin.