Sometimes, Revealing another persons gifts can harm that person. Especially, if they do not believe in such heresy. Life long Christians are taught from an early age about the occult and how everything which is extra ordinary is of the devil. For some reason the christian never makes the connections that Jesus exercised in the extra ordinary and actually gave permission for all of those who believe to exercise in those gifts as well. (John 14). A good sign of non-belief within the individual churches is where there is zero miracles being worked. Who believes and who does not is not the point of this particular posting. It is causing harm which is of importance and how not to harm anyone.
Another reason why you should not reveal gifts. Is because of the attention you should receive from others. The revealing can cause your pride and ego to bloat.