You make a good point, one I thought of after I posted. First, there are places that prohibit concealed or unconcealed carry, so the legal problem would be the same. Concealment would be preferable for 2 reasons I can think of. One is minor-weapons retention. It is harder (and less tempting) to grab a concealed weapon. Second is the deterrent factor. If not all pilots chose to carry, you would not want observers to know which ones chose to carry and which ones didn't.