Why should a belief be true just because it's useful? Or are you saying people are claiming a belief's usefulness is not true despite the evidence that it's useful?

Neither. I'm saying that a popular attitude of LW culture is to prefer not to "believe" the thing it's useful to believe, if there is any evidence the belief is not actually true, or often even if there is simply no peer-reviewed evidence explicitly associated with said belief.

For example, self-fulfilling prophecies and placebo effects. Some people here react with horror to the idea of believing anything they can't statistically validate... some even if the belief has a high probability of making itself come true in the future.