I've had 2 situations where cameras started to have shutter problems while I was travelling.
In Mexico, my OM4t, about 8 years old began to have intermittent shutter problems, which worsened after my return. I was also using a Mamiya 7 on that trip. Because the problem was intermittent, I did continue to use the Oly.
On a trip to Buenos Aires, I had two Epson R-D1s. The Epsons would have been 4 to 5 years old. One began to have a few blank frames. Again, I was able to continue to use it. The problem seriously worsened about a month after my return when the bad frames outnumbered the good ones. I also had a Ricoh GRD2 with me on that trip. Ironically, a few months later both of my Epsons had shutter problems simultaneously, and the Ricoh was my only working digital.
In neither case was one truly a backup, but it does reinforce the idea of always traveling with more than one camera.