I take a spare bass and an amp (if we're the only band on) to gigs, not practice, in the back of my mind we could use it as a PA amp in an emergency, once the guitarists amp packed up (or so he thought, in the end it was just power light bulb that had blown 😂), he had to use my spare, not ideal but got him through the gig.
We were on a multi band gig once, 30 minute sets, the bass player in one of the other bands broke a string, refused the offer of another bass, spent 5 or 6 minutes replacing it, completely ruined the momentum of the set