I've had basses go down on stage, after sound check, and have weird things happen enroute to a gig (play, all is good, take it out of case for sound check, and something isn't right.)
I carried 3-4 basses when I toured professionally. The last two years I toured with an artist I played around 700 shows. Traveled all over the country and in neighboring ones as well. I know most don't play at those extreme levels, but I've had everything from broken output jacks, cracked nuts, to strap screws holding strap locks break on stage during a show, or on the way. This was with maintenance on the equipment preformed constantly.
I still bring two basses to a gig, even though I haven't had any problems in the last year. I did have a bigger show where I only had one bass with me, and the output jack decided to get an intermittent short it in 20 minutes before show. Grr. That was the last time I ever brought just one bass to a gig.
For me, it's cheap insurance to avoid some serious headaches when the gremlins prevail.