Holy is love, holy is life. You will come to learn this mantra on WHM.

However, to actually help you, the relative mp cost: max mp ratio on holy is more or less the same at lvl 50 as it is at 60. But with experience, you will learn to not be too bothered about the cost when you get mana management down better.

Holy will let you dps a bit unhindered in dungeons due to the stun. The stun prevents damage, so its healing saved. Used alongside Aero III and Assize, its fantastically scary how much aoe burst dps a healer can do.

The useage of Shroud of Saints sensibly helps offset the mana cost of holy during dungeon pulls. Use it when holy spamming alondside assize for the damage and busrt dps.