A common objection to critiques of psychic and parapsychological claims is that neither research into them nor belief in them really does anyone any harm does it? After all, if we are talking about unlikely, but ultimately harmless, beliefs, then why bother making a fuss? Let people continue with their notions about telepathy if it makes them happy and gives them something to do.
Sometimes superstitious beliefs can actually be useful. For example, when an athlete believes in the power of a good luck charm, having that charm might impel him to work even harder than usual, resulting in a better performance. This usefulness of superstitious beliefs only goes so far, though, and only counts as long as such superstitions are carefully rationed.
Beliefs such as these are not always as innocuous as some presume. There is, of course, the matter of wasted resources after so much time spent looking for evidence of psychic phenomena and so little to show for it, it is not unreasonable to argue that the time and money would go to better use if directed towards more fruitful topics.
On the other hand, the mere fact that a topic is not going very far very fast is not, by itself, enough to forgo any and all research. When combined with other matters, like the a priori unliklihood of these claims ever having any validity, it would be enough effectively argue against bothering with most such research but it would not be enough to say that it should all be discontinued.
The question of how these beliefs can affect people, however, is a bit more serious. Psychics of various sorts and using various methods have been known to tell people that they have medical problems or that they have relatives who are in danger, none of which is actually true. They cause worry and concern which themselves can result in psychosomatic illnesses. In essence, claiming that a person is ill could actually make them ill.
When there is no real basis for the claim that a person can discern anothers medical condition through alleged psychic powers, it is unethical and irresponsible for them to make such a claim and then go ahead and try to tell people what is wrong with them. People with health concerns should see a licensed physician, not an unlicensed and unreliable psychic.
The fatalism which some so-called psychics engender can also be psychologically dangerous. For example, the book The Secrets of the Hand by Maria Gardini, an explanation and defense of the art of palmistry, argues that reading a persons palm provides not only highly accurate, but also unalterable information about the future a persons fate cannot be changed. Thus, part of a palmists job is to help the client psychologically and emotionally prepare by accepting the inevitable.
Does parapsychology do any harm? Any false belief might be innocuous or it might be harmful. At the very least, false beliefs can do harm by diverting our attention away from reality. This prevents us from properly dealing with what is true and factual for example, a person who goes to a psychic and is told to take herbs is diverted from the reality of their condition, a reality that a licensed physician could explain to them and help them with.
False beliefs can also lead us to fundamentally misunderstand reality. The belief that our fates are controlled by the stars or some other mysterious force may prevent us from taking risks, trying new things, or even just attempting to improve our lives in some fashion. After all, if our destinies are already mapped out in the heavens or on our palms, what is the point? What will be, will be, and there is no need to try to change whats going on.
Parapsychology probably doesnt have to do any harm, but in the end thats exactly what it appears to do anyway. Parapsychology makes claims about the nature of reality and because of that, it matters a great deal whether any of it is true or not. We need to know what is real and what is not because, in the long run, thats the best way for us to avoid harm and live better lives.