Back many, many years ago I belonged to a group called the "Boy Scouts" whose motto was/is "Be Prepared" which I have found, over the years, to be good advice. Having a back-up camera, to me, is heeding that motto. Can't do it with all my cameras but a camera with four lenses, most of the time equals four cameras, of course, as long as the camera body works. If you are using a 4x5 Graphic and a shutter fails, you put another lens and shutter in its place and continue shooting.. A big part of "being prepared" is to thoroughly check your equipment before going out.......Regards!