Halfus. no saving it wont matter, use it right off the bat then as often as you can.There's more to it than just getting the most mitigation out of it. If burst damage on the tank comes in physical and magic forms, Holy Shield can be used to mitigate the physical portion. Nef's Electrocutes come to mind here. Electrocutes alone won't kill a tank, it's an Electrocute combined with melee swings, or possibly a breath. The breath + Electrocute scenario obviously has no place for Holy Shield, but Electrocute + melee swings can be helped by the new Holy Shield.
temple. only matters when you jump back to nature to tank his ultimate, and you would be off cooldown most likely anyway, so spam it the rest of the time would be the same result.
mal. spam it if you are adds tank, spam it if you are main tank.
cho'gall. I can see this one, its useless during flames orders, so save it until shadows orders.
nef. useless to save it, spam it.
atramedes. useless to save it except in air phase, spam it when in ground phase.
val and ther, useless to save it except during transitions, spam it.
council. useless to save it except during transitions, spam it.
Cho'gal's Flame's Orders can kill a tank, and about 30-40% of the damage you take during them is from his melee swings, the rest fire damage. Being able to mitigate some of that burst through the proposed Holy Shield is wonderful, even if it's only a small portion. Cho'gall really doesn't do any serious damage outside of his fire/shadow/MC buffs, so the downtime really isn't a huge concern except for the fact that you won't have it up for the other elemental.