I love Banefire.
If someone with Decay gets the Diseased affliction, he will lose 5 HP (non-Regen'able) per turn for the rest of the battle. Usually a human who gets Decay from Banefire will be dead within 5-7 turns at most, which makes the useful AoE of Banefire much larger than it appears from the spell description.
Obviously this is more useful when you're facing humans/monkeys, and pretty useless when you're facing vans/undead/summons.
-Max