I don’t see any reason people can’t believe things they also see as lacking a purpose
Because believing in something—really believing in it—is not costless. It comes at the cost of those other beliefs incompatible with the one in question. This doesn’t make it impossible to harbour beliefs without any useful purpose, but it’s a reason to expect to to be uncommon. Should an idea be incorrect merely because it’s uncommon? No; but if it’s both rare and intrinsically unappealing—lacking both the force of reason and the weight of mass assent, why then should it be taken seriously?
Because believing in something—really believing in it—is not costless. It comes at the cost of those other beliefs incompatible with the one in question. This doesn’t make it impossible to harbour beliefs without any useful purpose, but it’s a reason to expect to to be uncommon. Should an idea be incorrect merely because it’s uncommon? No; but if it’s both rare and intrinsically unappealing—lacking both the force of reason and the weight of mass assent, why then should it be taken seriously?