Sanctuary. It makes the raids and instances go faster, and even if it is worse against a single target than kings, and even if both of our main healers who happen to be paladins specced into kings didn't have it, .... I'd still buff sanctuary rather than gimping my spec by draining 5 talent points into kings for a slight bit extra mitigation situationally that I'd be better off getting with those points elsewhere in the tree.

Edit: Besides, the surplus of mana on bosses I use to put sacred shield up on myself that I might not otherwise feel free to do. I wonder how that factors in when you take mana regains into account.
_________________
-Bringer of prot-itude on the Shu'Halo realm-