Though the zombies were exorcised in the sunlight, their rotting bodies remain, and can be used to spread disease in enemy territory, or as material to fill up holes, or recycled as meat for Hobufu-chan who liked to eat them, but ghosts who leave nothing behind can't be used at all once the sun comes up.
Besides, it also hurts that they can be easily exorcised, specifically by using holy water or holy light.