Actually, with Shield currently being less efficient than both Heal and GHeal, I believe tank healing disc priests should configure their UI to show Rapture ICD, and only shield the tank when the ICD is up. This makes more than one point in Strength of Soul pointless.
That's a valid point, and if you follow that line of thinking through, then we're currently only shielding the tank for more throughput* during tank-pounding phases (in which case we won't be using Heal in our single target rotation anyways), or for keeping up the Weakened Soul debuff.
But if that's the case, then the points in SoS are at best only usable for making optimal use of Rapture ICD as pointed out by Elerion, but at worst can actually be working against us (i.e. forcing us to re-apply WS debuff on the tank earlier than otherwise, with no net MPH or HPS gain to our rotation).
Perhaps it's worth considering if we should be so interested in reducing the WS debuff in the first place, if we're really only using shield for
1) absorbing heavy incoming damage,
2) As a sort of emergency heal when people are at low health,
3) maintaining the WS debuff for 5% crit,
4) Proccing rapture (Which "only" makes it a nearly mana-free absorb, not a net mana return as we're used to seeing)
Question is, in how many of those situations would there be a WS debuff on the target when we need to apply a shield?
*More throughput in single-target rotation due to shield on tank + BT hasted penance, pom, gheal is more throughput than a similar use of BT where the shield is thrown at an off-target because the tank had WS up)
Sorry if I'm just speculating here without math to back it up, but it seems to me that in practice, there really is no net gain from SoS. Or am I missing something?