I just used to take spare strings to gigs, but for the sake of carrying one extra thing out of the house, if a string goes mid song, I can just grab the other Bass off the stand and crack on. Saves any downtime.
In our cover band, we do a lot of parties / weddings / events and on those sort of gigs, the clientele can be less forgiving of downtime than the local punters at the dog and duck.
I still take a backup for Pub gigs though, it's just habit for me now.