Alright, I've thought about your interpretation of Ouramancy, Misa, and come up with an alternative:
You say that it is an evolutionary response, something to allow these families to survive their multiple encounters with Spirits and Magical Beings... And I don't quite agree. To me, the biggest distinction between Ouramancers and the common people is exactly what Hanged has just said: "Every other kind of Spirit is as you think - only an Ouramancer can communicate and interact with them."
So, there are gods that have an interest in this world, there are the spirits of ancestors and of the simple dead, and they have a single conduit for effective communication with the physical plane: Ouramancy. It isn't an evolutionary response on the part of the humans, it's a path laid down by the Divines themselves. They have crafted Ouramancers as their connection to the physical world, gifting what would have been ordinary magestravi with these altogether unique abilities. After all, how could a human, magical or no, learn to speak with the gods, if that knowledge didn't come from the gods themselves?