A little anger goes a long way, especially if the Vengeance of Stone and Blood punishes the transgressions of your enemy. Spoils of War are a nice way to make those transgressions a bit more attractive to him. Once the battle gets started for real, Grombrindal and his Elite will incite the dwarven war passion. And since enraged dwarves don't care much who actually burned down part of their capital, those strange Temples of Vaul will be useful in more ways than just one.