There's a story I heard from a friend at a recent Rationality meetup. It goes like this:
When Europeans were colonizing Africa, they told some Africans that they had to move their city. Their city was on a plains, and Europeans wanted a nice city like those at home: on a river. The Africans objected, saying that they couldn't live near the river. That's where the red spirits were, and people would suffer if they lived there. The Europeans made them do it anyway, because red spirits clearly don't exist. And then everyone got Malaria.
I think there are two thing going on here:
1. The colonizers were basically assuming that the moral of a fairy tale wasn't useful because the fairy tale wasn't true.
2. The Europeans were ignoring a story because it didn't fit in with the terminology that they already used to describe the world.
Fairy Tales With Morals
The colonizers assumed that, because the justification for a custom was contradicted by scientific understanding, the custom wasn't valuable. Red-spirits don't exist, so there's no reason to follow the custom.
The issue with this is that culture is subject to evolutionary pressure in the same way as genes. Cultures that lead to their adherents prospering are more likely to be present in the future, so any currently existing cultural artifact should be assumed to have served some important purpose in the past. That purpose may not be clear, or it may not be one that you agree with in a moral sense, or it may not apply in the present, but the purpose almost certainly existed.
This is basically a Chesterton's Fence argument at the cultural level. If the colonizers hadn't assumed that something they couldn't see a reason for had no reason, many people's lives could have been saved.
The terminology mistake is, in my opinion, even more dire. The colonizers argued that red spirits didn't exist, so people should move to the river. My friend who told me this story argued that the native villagers were mistaken for believing in red spirits, and that they should instead have believed in mosquitos.
The problem is that it isn't clear from the story that there's any difference between believing in mosquitos and believing in red spirits. Maybe red-spirits just means mosquitos. Or maybe it means malaria. The story doesn't have enough information to tell if the villagers were actually wrong about anything. When I brought this up, my friend couldn't answer any questions about what believing in red spirits actually meant to the villagers.
This is a failure mode that I think is common to people who describe themselves as scientists. I've noticed that people who describe observations in a way that doesn't use standard scientific jargon are often dismissed by people who are super into science. That happens even more if the description given uses words often used by marginalized sub-cultures.
People may be describing the exact same observations, and using the same model to describe those observations, but argue because they're using different terminology. It seems important to actually try to understand the model people have in their heads, and try to avoid quibbles about how they describe that model as much as possible.
There's another level to this if you assume that red spirits actually means ghosts in the western sense. Science-afficianados like to talk about the value of testability, but both mosquito and ghost models are testable. If you think that mosquitos carry tiny cells that can reproduce in your body and make you sick, that implies certain things you can do to prevent disease. If you think that ghosts get angry if you live in a certain area and make you sick, that implies other methods of prevention. People can try these prevention methods and see what works; they can test their theories. Just going in and saying that ghosts don't exist totally ignores any tests that the villagers actually did before you got there.
The Use of Red Spirits
Even assuming that red spirits literally meant believing in ghosts, that idea was saving lives at the time that the colonizers moved in. It seems like there are a lot of fairy tales like this: explanations whose constituent parts don't correspond with things in the real world, but that still accurately predict patterns in the real world.
I think that this is the source of a lot of cultural relativism and post-modernism. If someone thinks only of the outcome of explanations, then the actual truth value of the component parts of the explanation don't matter. All explanations are as valid as they are useful to their culture. Since cultural evolution strongly implies all stories and explanations serve some useful purpose, every story a culture tells is useful. Therefore all explanations are true.
The only mistake that I see with that is the idea that a useful fairy tale implies that each component of the fairy tale is useful.
Thinking that mosquitos cause malaria implies that you should avoid mosquitos, which (as we now know) can actually prevent you from getting sick. Thinking ghosts cause malaria might be useful if you end up avoiding mosquitos while also avoiding ghosts. Given that avoiding ghosts leads to avoiding mosquitos, the main reason to prefer one of these over the other is if one is less onerous.
Beliefs and stories rent out a share of your brain by being useful to you. As the landlord of your brain, it seems like the best thing to do is get beliefs that will pay you a lot in usefulness while requiring little mental real estate for themselves. Believing in and acting on the mosquito-malaria connection takes a certain amount of mental effort. I'm not sure what a belief in a ghost-malaria connection that actually led to avoiding malaria would entail, but I can guess that it would be more mentally costly than the mosquito-malaria alternative.