Note that Zealot's Fire is cast on you; the effected foes are adjacent to allies that you target with skills subsequently. Thus Zealot's Fire is most effective when targeting, e.g. healing, allies in close quarters with the enemy such as warriors and pets that are dealing or taking melee or touch damage.
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 the targeted party member, making it a useful spell for primarily non-offensive monks.
Combine with your skill sets as a 55 Monk to deal more damage to your foes if Shield of Judgment isn't able to do it on its own.