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 π), 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