Fights like Garrosh where you have to run some what around, its nice to have everyone shielded up before imo.. them 200-400k shields really do wonders, and its way better than having a bit more crit and not using pw:s outside of LMG. So thats kinda why im staying at 10-12k spirit. I rather do a bit less atonement dmg/divine aegis which tends to not getting used, and still save my raid or make the fight a lot easier to survive due to pw:s or spirit shell.
Thing is i can always reforge away from some spirit if i need it on fights where on dont do much but atonement, to fights where some pw:s spam from time to time can save the raid or make it easier.