The stats are all wrong
In order to play a DPS spec optimally, you can't have a pet stat. You choose whatever stats are going to give you the best results. When you're playing a healer, things are a bit different. Healing can't be done by the numbers, and there's leeway for various playstyles to emerge. With the addition of mastery, healers have an even more diverse set of stats to manage. Perhaps raid healing favors haste while tank healing favors mastery, and you can adjust your gear to accommodate your role. Healers get to be flexible.
With all of that flexibility in mind, our Reinforced Sapphirium Regalia is still awful. None of the set pieces have any haste on them, compared to the two pieces of Lightsworn that had haste. While there is some discussion to be had on whether haste or spirit is the better stat for holy, the fact that haste isn't even present makes every piece of gear suboptimal. If you flash back to our ICC days, you'll remember that our best-in-slot setup featured several slots filled with mail armor. We didn't want to wear mail, but there simply wasn't any good plate available. We don't have that option due to the new armor specialization. By itemizing all five pieces of our set without haste, we're pushed toward whatever other plate we can find.
Haste simply scales better than any other stat for us right now. The fact that haste can add extra ticks to Holy Radiance is one of the big reasons, and how useless critical strike rating is another. Haste is tops when it comes to increasing your throughput. Since the removal of Illumination, spirit is the only secondary stat that helps with your longevity. I like to think of Divine Light and Flash of Light as a healer's version of Life Tap; we can convert mana into extra healing by using a bigger heal. Spirit helps us maintain enough mana to unload these bigger heals when necessary. The rest of the secondary stats simply aren't that attractive. We're looking for gear with a mix of haste and spirit, and there's not a single piece designed like that in our set.
The set bonuses are weak
If you put a good set bonus on a badly itemized set, it can make the set worth picking up. Unfortunately, neither of the set bonuses on Reinforced Sapphirium Regalia are good. With Holy Light only doing around 10% of a holy paladin's total healing in a raid environment, buffing its critical strike chance by 5% is simply not that valuable. Critical strike chance is not that great for us already, and by buffing one of our weakest heals, the two-piece bonus is negligible. Surely if the set is itemized poorly and the two-piece bonus is bad, the four-piece bonus must be simply amazing, right?
Wrong. The four-piece bonus grants us about 2,000 mana every time we use Holy Radiance, which is only around 20% of the cost of the spell itself. Over the course of a 5-minute fight, assuming we use HR once a minute, that's only 10,000 mana returned. It'd be more efficient to spend your valor points on a pair of BoE boots, sell them, and then buy a ton of Mythical Mana Potions. Two Seal of Insight procs would match this bonus, and even working in an extra Judgement will have you getting more mana back. It might have been slightly better back when Mana Tide Totem was based off of our spirit, but that's no longer the case.
We can do better
There's not a single well-itemized piece in the set, and the bonuses are laughable. On top of both of those deficiencies, there's yet another nail in the coffin: better options. There are other pieces of well-itemized plate healing gear, and they drop from relatively early and easy bosses for the most part. The shoulder slot is the only one without a great option available, though even the Burden of Mortality from Chimaeron beats the set shoulders that require a token from Cho'gall. Because holy paladins are the only spec that uses intellect plate, the non-set items are going to go to us anyway. We might as well use our valor points on other items and save ourselves the trouble.
The Light and How to Swing It: Holy helps holy paladins become the powerful healers we're destined to be. Learn the ropes in Cataclysm 101 for holy paladins, study the new balance between intellect and spirit and learn how to level your new Sunwalker. Tanking is a job, DPS is a craft -- but healing is truly an art.