This can be a useful skill for any Healing or Protection monk who has some spare points to go on Smiting Prayers. Since Zealot's Fire will do damage to any foes adjacent to your target it's often best to concentrate on healing warriors and pets, since they will most likely be closest to your foes.
Unlike most other Smiting Prayers, Zealot's Fire does fire damage instead of holy damage. This damage is not armor-ignoring.
Monsters will flee the damage caused by Zealot's Fire, but only if the damage is triggered by spells cast in succession. This can be exploited to drive monsters away from certain teammates.
This spell is useful in PvP Random Battles as it will significantly improve the amount of Faction you receive by causing light damage to several opponents. (Healing monks often receive less than their share of faction because they don't normally deal damage)