It sometimes seems as though skeptics and atheists constantly face being put on the defensive by the question "but where's the harm?" We usually cite some of the many atrocities committed by varieties of believer or scammer. But in fact, the question confuses me. In what way could anyone consider that a world in which people routinely behave irrationally would be better than one in which they do not? The benefits of rational thought and the costs of the irrational are simply self evident and I'm not sure this question even deserves an answer.
Compare this to the answers generally given to the opposing case: "where's the harm in skepticism?" The claimed consequences of skepticism are almost always dire (hellfire, chronic ill-health, death of oneself or a loved one through failure to pass on a chain letter etc.) and always need to be meticulously spelled out. This is precisely because it is not rational that these consequences should follow this cause.
I suspect that people really only ask this question because answering it forces others to cite specific examples, which can always be argued against ("yes, but the church does lots of good, too" or "yes, Uri Geller might have been conning us all for decades, but that nice Silvia Brown is the genuine article.") This makes it seem as though the questioner is being magnanimous in conceding that there might be one or two cases where something bad has come from some brand or other of nonsense, but that the skeptic is unreasonable in suggesting these scant examples accurately summarise that whole area.
In the future, I'm not going to allow myself to fall into this trap. I think the correct response to the question is to throw the ball back into the questioner's court: why are they suggesting that irrationality is a virtue?