I was under the impression that we should be using SoR for multi-target fights. Does anyone know the exact number of mobs where SoR becomes better than SoT? I've assumed it was 3, but I'm not actually sure.
If you had no reason to use GCD, then the answer is 4 targets. At least 4, otherwise Divine Storm isn't granting HP and you'd be better sticking with the single target setup. Using SoR and CS at 3 targets is a worst of both worlds scenario.
Since you do have to use GCD it becomes more nebulous. You are going to exchange 2 GCD (SoT->SoR and back when finished) in order to swap. Someone could crunch the numbers on precise timing, but if the targets die in less than about 20 seconds, you're probably not gaining much, if any, DPS. In the 20-25 second range you are simply redistributing your DPS from a single target to all your targets equally (losing 2 direct damage GCD for the extra seal procs). 30 seconds or more with 4 targets (boss and 3+ adds) would be about long enough to see a total DPS increase.
Additional targets beyond 4 may shave off a half-second to a second, each. Remember that if you drop to 3 total targets, you're suddenly under threshold, so you need at least 4 adds alive into the 20-30 second range in order to gain the benefit.
I understand SoR could be useful during Heroic Madness (I have not experienced the fight), but I'm hard pressed to think of another situation in Dragon Soul where SoR would be advantageous. Yor's adds die within the loss/break-even timeframe and AOE is either unnecessary (normal) or undesirable (as I understand Heroic) on Spine.