That's rather interesting. With Holy Reach, PoH attains a reach of 36 yards, almost the same as the standard 40 yards healing range. While there are other spells that have similar effects (Holy Nova and Circle of Healing being the most obvious candidates), none other have such a massive range.
Since you have a wall between you and your healing target, it's almost always safe to blow a full cast-time spell. There's two questions still waiting, though. Does it have enough healing throughput (hps)? How's the mana efficiency?
Let's look at the last question first. Warning, theorycrafting incoming.
PoH efficiency and throughput
Let's look at the best case. The priest in question is specced Holy (which isn't such a bad idea, now that damage is through the roof in PVP. Blessed Resilience is more useful than ever). I include the following talents:
- Healing Prayers: Boosts mana efficiency by a whole lot.
- Spiritual Healing: 10% more healing done. More healing = more efficiency and more healing throughput.
- Divine Providence: 10% additional more healing done, at least with Prayer of Healing.
Test of Faith is also a nice talent for this purpose, but doesn't always apply.
I have no idea how much spellpower a lvl 80 healing PVP priest packs, but I assume it's somewhere near 2500, including talents and abilities such as Spiritual Guidance and (Improved) Inner Fire. That is a pure guess, but please entertain me with the thought. That will increase all healing spells by up to 1.88*2500 = 4700. Pretty hefty number.
Now, Prayer of Healing's base healing is (2091+2209)/2 = 2150. A 3-second cast-time spell receives 3/3.5 of +heal, unless the rules have changed. It gets penalized for being an AoE healing spell, divided by 3. Thus, it gets a bonus of 4700 * 3 / (3.5 * 3) = ~1343, for a total of 2150 + 1343 = 3493 before talents. Talents modify this, 3493*1.1*1.1 = 4227 average healing per target.
Base cost is 48%. With Healing Prayers, that's reduced to 38.4% base mana. Healing Per Percentage of Base Mana (hppbm) = 4227 health per target / 38.4 % base mana = 110 health per target per base mana. Healing throughput = 4227 health per target / 3 sec = 1409 health per target per second. The "per target" part is interesting.
I choose to compare it to Flash Heal. Part of the reason is that I know of course that nothing will ever come close to Greater Heals mana efficiency given you've picked up all the talents boosting it. The other part of the reason is that Flash Heal is the most likely spell you'd pick for that purpose, apart from ducking around the corner to tossing some instants.
I'll evaluate it to the best and worst case of Flash Heal.
Flash Heal best case
Best case: Maxed out Empowered Healing and using the Glyph of Flash Heal.
Flash Heal, average (1887 + 2193) / 2 = 2040. Incrased by 4700 * 1.5 * 1.2 (Empowered Healing) / 3.5 = 2417. Total of 4457. Further incrased by Spiritual Healing to 4457 * 1.1 = 4903.
Mana efficiency: 4903 / (18 * 0.9 (the glyph)) = 303 hppbm. Compare that to PoH's 110 per target per percentage base mana, and it's almost three times higher on a single target. On two targets, it's 303 vs 220. On three targets, 303 vs 330.
Healing throughput: 4903 / 1.5 = 3269 per sec. Well over twice PoH's 1409 per target per sec on a single target. Getting close at two targets, 2818 per sec, and miles behind on three targets, 4227 per sec.
Flash Heal worst case
No Empowered Healing, no Glyph of Flash Heal.
Flash Heal, average (1887 + 2193) / 2 = 2040. Incrased by 4700 * 1.5 / 3.5 = 2014. Total of 4054. Further incrased by Spiritual Healing to 4459.
Mana efficiency: 4459 / 18 = 248 hppbm. Significantly worse than best case. Compare that to PoH's 110 per target per percentage base mana, and it's still a lot better to use Flash Heal on a single target. On two targets, it's 248 vs 220, pretty close. On three targets, 248 vs 330.
Healing throughput: 4459 / 1.5 = 2973 per sec. Still twice PoH's 1409 per target per sec on a single target. Very close at two targets, 2818 per sec, and again miles behind on three targets, 4227 per sec.
There's one other obvious comparison we ought to include. If you're already planning to heal yourself and one other target, we have a spell for that purpose...
There's one problem here. I have no clue how well Binding Heal scales. Identical to Flash Heal? Half as good? I'll just have to assume it scales identical to flash heal.
Quickly, best case: (2230 base avg healed + 4700*1.5*1.2 (Empowered Healing)/3.5)*1.1(Spiritual Healing)*1.1(Divine Providence) = 5623 per target. Since it heals exactly two targets, 11246 healing done.
Mana efficiency: 11246 / 27 = 417 hppbm. Healing throughput: 11246 / 1.5 = 7497 (!) hps.
Worst case (still with Divine Providence, since I'm comparing to PoH): (2230 base avg healed + 4700*1.5/3.5)*1.1(Spiritual Healing)*1.1(Divine Providence) = 5136 per target. Two targets = 10271.
Mana efficiency: 10271 / 27 = 380 hppbm. Healing throughput: 10271 / 1.5 = 6847 hps.
Working as intended
Flash Heal is much better on a single target. Binding Heal is a heck of a lot better on two targets if you're one of them. Then there's also the risk of "missing" because unlike targeted heals, you don't get any error message if the intended target is out of range.
That doesn't change the fact that you can surprise someone with this trick, but it remains very circuimstancal.