If a criminal is going to commit a robbery in the presence of an armed person, it is better for the armed person if the criminal not be aware of his armament.
On the other hand, most criminals have more sense than to knowingly commit a robbery in the presence of an armed person.
I guess a fair question would be whether it is better to carry concealed and have a criminal attempt his crime in one's presence, or to carry openly and have the criminal wait until one has left.