I never take backup bass to a local club gig, but I do have a spare strings set, cable and batteries. I figure it wouldn't be a big deal to put on spare string if one failed. On a biger gig, like festival, I may bring backup bass, even though I haven't had any kind of failure in more than 20 years of playing.
In the history of modern music, most bands played with members having one instrument and making do, while creating music we admire today. I fail to see why would someone playing local pub equip themselves on par with some big name pro on a continent tour.
Edit: Further more, never saw key player bringing backup. Same as drummer, or anyone from horns section. When I think better, only guitar players, having instrument prone to breaking strings due to bending, carry backups.