Agree.. in the day of the shoot anything may go wrong, but equipment failure should not be one of them.
Key word here being "should"
And as the way the brain works, it's always at that "perfect shot" moment that the blasted thing breaks down... and then you go,"Oh man! Why? Why? Why?! That was the perfect shot! Stupid camera!"
If on a paid assignment, why chance it if you can afford to bring a 2nd body, whether as backup or simply to have 2 bodies with 2 different lenses to cover the range immediately.