Assuming a single damage pulse arrives that takes players to low enough health that a PoH fully heals them, you get 0.6 in healing and 0.4 in the shield for X mana. N more PoH's with no healing but only leaving behind a shield is 1.0 + 0.4 * N. The more PoH's you cast, the less efficient the entire chain becomes. Now it is true that if you don't cast the PoH to sustain the DA that you lose it all, which leaves the efficiency at 0.6/X HPM. If you need one PoH to bridge it until the next damage pulse, then the efficiency is 1.4 / (2X) or 0.7/X. Barely more efficient than simply letting the DA expire. Two PoHs gets you to 1.8 / (3X) or 0.6/X -- this is where you were when the DA would have expired on its own. Three bridging PoH's and you're at 2.2 / (4X) or 0.55/X HPM.
Your math is incorrect in how you compute efficiency......So it isn't efficiency.....Spirit Shell instead, which is far, far, far higher efficiency *and* HPS than using DA for it.
Smite is certainly not hugely efficient....
The style you suggest is not likely to be effective in a mana-starved situaiton (which is what we will all be in).
My calculation is correct. I am just using % numbers to compare. You calculate the exact same thing: I am averaging the % of the amount healed and dividing by the number of spellcasts. Its practically the same thing, no point in arguing semantics.
I gave you a very specific example with spirit shell. After spirit shell is absorbed you still have people to heal so you will have left over aegis. I have shown that its potentially worth keeping it alive using the minimal number of PoHs. It will increase your HPS dramatically, and it can even boost your HPM [edit: depending on the overheal you get from PoH. Spirit shell is a 1 minute CD and you will still cast no spirit shell pohs after the raid stops taking damage during the top up, so again you have aegis worth keeping alive. Unfortunately spirit shell does diminish the value of aegis stacking (if its up) in cases where all spirit shell must be applied before the spike hits (i.e. spike is only 1 hit). If that is the case its a really bad fight for disc.
This is one of the major issues disc has at the moment. PoH without aegis being absorbed has abysmal efficiency. Hopefully blizzard will understand this and let spirit shell refresh aegis, but we will see.
Remember that PoH can also overheal, which increases the value of the aegis you have stacked. 70% to 65% is a drop of roughly 7.2%, but that is with zero overheal on the original PoH. Since aegis is a 15s buff you can potentially (safely) extend to 25s after the intial stack is established (not when the damage hits but the actual first aegis stack is established). If you have cast 2 PoHs on a group, The you can extend it for roughly 25s with a gain in HPM, or you can extend it for 37s with a 7.2% loss in HPM
Basically we can conclude that I am right. As long as you don't need to cast a lot of PoHs, once you use it to heal a spike its worth keeping aegis alive for a certain amount of time depending on the left over stack. If the fight allows you to do that it makes sense to do it.
PoH = base*(1+crit*(1+0.9*(1+mastery)+0.3(1+mastery))
% of total heal from mastery = (crit*0.9+0.3)*(1+mastery)*100/(1+crit)
Adding in 5% crit and 30% mastery this comes up to 42.71%
The % of aegis is 42.71/1.4271 ~ 30%
Crit increases that value quickly because PoH crits are roughly 50% aegis.
In raids we can have 37% mastery and probably 10% crit due to the crit buff. In that case it would be 32.7%. With that Casting a full healing PoH and then a 100% overheal poh to keep the aegis alive has an efficiency of 132.7/2 = 66.35%, nearly the same as letting the original PoH expire (67.8%). However even PoHs that are not cast with the sole purpose of keeping aegis alive can overheal, sometimes a lot, which greatly raises the value of keeping your aegis alive.
[edit2] With 470 scaled gear I have 12.55% crit and 47.78% mastery, which brings the aegis percentage to 35%. This means casting one extra PoH to keep aegis alive increases your HPM.
about 60-70% of our heals is PWS, aegis and spirit shell, making mastery a very good secondary stat for disc. Aegis stacking is going to be even more important as our gear improves since mastery and crit will go up.
Summary: Keeping aegis alive is crucial to maitaning both high HPS and can even be a gain HPM depending on gear/conditions.
If you don't agree I am happy to run a little simulation for you. Serrif pointed it out correctly that I am wrong about the amounts. Its 42% of the actual heal amount not 42% of the whole PoH. So its 42% of the heal, which means its 0.42/1.42 = 30% of the PoH value. I will revise the calculations.
The correction means that you can use less PoHs to keep a stack alive, but the results are qualitatively similar. The inefficiency of using PoH to stack aegis is mitigated by the fact that it keeps aegis you stacked by healing alive.