Believing that laetrile can cure cancer can be fatal. Believing that a spaceship is hiding behind Comet Hale-Bopp and is waiting for humans to come on board can be fatal. Believing that prayer alone can heal children with type 1 diabetes can be fatal. Believing that there's some invisible superpower who commands the killing of those who don't believe in him and who don't follow his supposed orders can lead to fatalities. Believing in falsehoods can be and not infrequently is harmful to the believer and/or nonbelievers.
Therefore, respecting beliefs indiscriminately is patently unethical. To respect beliefs blindly and equally is to be party to evils that can arise from them. Just as we are ethically bound to proportion our belief according to the quality and amount of evidence, so we are ethically bound to proportion our respect for beliefs of others according to the truthfulness of those beliefs.