Holy is so useful not only does it rip through trash mobs and adds it is an extra stun for the party that works great as an emergency utility when somebody forgets to stun or you need another stun on top of one used. On a boss fight with no adds you'll find yourself not needing it over your single target attacks but it has many uses and even pre getting assize you have shroud so just learn to use that a lot. at about 90% MP pop shroud and by the time you need more MP it'll be off CD again. A lot of trash mobs won't last long enough for you to be out of MP.

You don't need to be constantly sat on 100% MP as much as your party doesn't always have to sit on 100% HP. It's about seeing the fight and knowing what you'll need.