Why is it useless? Everything is very relative in this world :)
1. Vampire Princes. With soldier's luck, the chance of putting an enemy to sleep during an attack increases. At the same time, maximum damage is dealt, the opponent does nothing for three turns, and if you attack him, it's as if he has rested and won't give up.
2. If you are playing against the AI, you will have a large army, and simply doubling the damage dealt during an attack will not hurt. :rolleyes: