And smite healing 'double dips" on crit. The smite can crit (double damage = double heal) and the atonement can crit from what I remember! Again a crit either way is a smart heal to the lowest health player!
Not exactly. It does not double-dip into crit, but it does double-dip into the crit meta effect gem. The Atonement heal is a crit exactly when the corresponding smite/hf/penance is a crit. However the amount of the heal has an extra 1.03 factor in it. It's not huge, but it's nice to have.
The main advantage of atonement healing, apart from its smartness which is a big deal, is how ridiculously effective it makes penance. The Penance of a priest who keeps Evangelism going is 20% stronger and 30% cheaper compared to that of one who doesn't. This combines to an about 70% increase in Penance's HPM. How anyone could give up on that I don't understand. I believe any Disc priest these days should at the very least be using holy fire on CD and Penance offensively some of the times at least, to keep the Evangelism stacks rolling, and provide Archangel on demand. I feel doing otherwise is gimping your character. Utilizing Evangelism is a must, not a choice. Full Smite healing is a choice.
There are effectively 3 "levels" to "smite healing":
1. Cast almost nothing but Smite, HF, Penance. This provides the maximum dps benefit, but also hampers your hps. Excellent for a third healer on a fight where 2 healers would be enough but you want to back them up a bit, but frankly have one of those two healers go dps and play a "full disc" instead for an average hps around 60-70k without breaking a sweat.
2. Cast HF and Penance on CD, Smite only rarely, as a filler. Use Penance both offensively and defensively depending on whether you need Grace stacks on someone or not. Utilize the full disc arsenal, with frequent PW:S casts, Spirit Shell, Cascade/other lvl 90 talent. Pop Archangel frequently, preferably combined with Spirit Shell, to enter god-mode. Don't shy away from using GH when massive single target healing is needed. This way of playing is what I call "full disc": Use all the tools provided to the spec. Much less dps than option 1, but considerably more hps, with a penance that hits like a truck. Downside: Too many cds to keep track of.
3. Avoid HF/Smite like the plague, completely ignore Evangelism/Atonement/Archangel. Easier to work with for people who have a difficult time healing one person while targeting another, fewer things to keep track of. Lower hps than option 2, no dps at all, somewhat better Grace uptime.
I would call way 1 "smite/atonement healing", and way 3 "non-atonement healing". This sadly leaves the one I recommend, 2, without a name, hence my choice of using the term "full disc". Whether we like it or not, The Atonement/Evangelism/Archangel package is part of being a disc priest these days, not any more an option that someone might choose to go into. We should not ignore it, nor should we ignore everything else instead.