my main amp is 20 years old and never failed in the many years I've owned it, but if it did I would still be into the PA so I could finish out the set before switching to a small BGW 100 watt amp in my rack that I sometimes use to power a monitor for our drummer. I actually had a jack on my cab go bad, as well as a speaker taking a dump, and well...I don't bring spare cabinets
I see people here who bring a "suitcase" full of this and that, and I have nothing against that as anyone should be prepared for the worst to an extent.
I also look at many threads here about "my SVT blew a fuse, my cords stopped working, my battery went dead, my J-pickup started buzzing, my strap felt loose, my shoes were too tight"...ok, that's fine of you want to bring a bag of tools, cords, batteries,fuses, straps, strings, 3 backup basses and an extra pair of shoes....
whatever makes you feel comfortable before you leave your house is your responsibility, so you bring what you must I suppose.