I appreciated the question "If a belief is unreliable but beneficial, why should we convince beleivers they are wrong ?"

True enough, if someone is cured by a placebo and there is no available cure, should we tell them they're on placebo ?

My answer would be "Yes we should, precisely because, in an educated society, they can figure out at any moment by themselves that this belief is unreliable". You wouldn't dare to cure someone with a placebo if they could read that the word "placebo" is writen on the pills.

As it is, to make sure the placebo works forever, they would have to prevent people from learning to read. The cure would be worse than the disease.

If someone justifies all their morals, metaphysics, social relationships, love, hope, politics, education, laws and justice, etc on something that can be prooven unreliable at any moment by critical thinking, they are building a pretty dangerous and unstable society if exposed to education and reason.