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!
In fact, the Motto of Boy Scouts of America still is "Be Prepared".
Just last week I went to pick up my son from his stint as staff at a Boy Scout camp, Camp Oljato.
I packed a single body OM-4 and a few lenses. Day was rainy and true to OM-4 temperament the humidity caused a malfunction of the electronics.
The battery check was good (I had 8 spare batteries anyway), and the display was right, but the shutter sounded fast. So I cranked it down to f/16 and aimed at something dark and took a shot, sure enough it did not fire the shutter at the indicated speed. I tried a number of different workarounds, winding and firing to see what might solve the issue. Changing ASA and exposure compensation finally did get the shutter to sound right, but by that time I had finished a roll of film with just test shots.
Disgusted with the day, I went back to my tent where, lo and behold there was a stowaway in the top of my pack.
A Kodak Retina I somehow got into my gear even though I didn't plan on any spare camera.
So there I was, I loaded the Retina and tossed the OM-4 with all its lenses into the bottom of my pack, and spent the rest of the time with a Retina in my pocket.