Things that have resistance to spirit damage: Ancient Guardians, Animated Wastes, Minotaur (all variations), Carrion Eaters, Devourers, Coral Snakes, Dragon Priest, Earth Elementals, Giant Chieftains, Giant Mambas, Great Manticores, Greater Demons, Hill Giants, Ice Giants, Ice Warlords, Insane Wizards (red and blue), Iron Golem, Iron Guardian, Kurg Elder, Kurg Silverback, ..... The list keeps going... Oh. gawd, the list has gotten so long.
Okay, so there's a decent amount of things that have SOME spirit resistance, but most of those have comparable or better resistance to other elements. But NOTHING IS HEALED by Spirit damage... So
Spirit Damage is a good all purpose damage type but it shouldn't be viewed as a cure all for weapon damage.