Note: Holy still does full damage even if you have the bare minimum (280 MP) to cast it.

The thing that makes Holy good is the fact that it's instant cast proximity AoE and deals a good amount of damage (more than Flare on BLM). If you've done a lot of stronghold fighting or attempted Cutter's Pass, you'll realize that the game tends to have a heavier focus on AoE when compared to XI because normal mobs die quickly.

Burning MP on non-comboed Stonera or Aerora along with BLMs before using Holy to finish off a group of mobs can be more helpful in the long run than being a cure bot. Too often I see a situation where a bunch of almost dead mobs are beating on BLMs that keep getting interrupted/have low MP while the WHM frantically burns MP healing them. You'd be better off nuking along with them before that stoneskin/sanguine rite buff fades.

With so many ways to regenerate MP now (Blissful Mind, Shroud, Ballad, standing still lol), I think more WHMs need to be more liberal with their MP and help kill faster. More dead mobs = less cc required, less healing required, saves time, etc. WHM (along with BRD) is almost too good in this game because they add so much utility while having decent and safe DD potential. I'd like to see more people take advantage of this.

Of course, this only applies to situations where you know that helping out will result in dead adds and that everyone can survive for 10 seconds without you healing. Don't blow your MP when you are the only healer, your MP regen abilities are on cooldown, and the mobs are nowhere near death.