I've never bothered taking a backup or needed one. I always have essentials within reach, strings, leads etc.. In the time it would take me to dig out the spare bass guitar and tune it I could have almost changed the string. My spare strings are my old ones so they are already cut to fit and it takes no time at all. I guess if I broke strings regularly I would but it's only happened once. Changed it really quick, no problems. After the gig half the band hadn't even noticed!
Keyboard players/double bassists/drummers/sax players etc... don't take a spare. The only time I see it is with regular guitar.