I’m a bit baffled by those who think that because they have the tools to fix anything, it’s problem solved. If your bass fails mid set, are you going to whip out a soldering iron?
Ive been gigging since 1980 and in that time I’ve only needed a spare 2 or 3 times, but it can happen. Once it was a wire that had come astray, mid song. Strangely enough I don’t tend to check the wiring (rather complex in that particular case) before every gig so hadnt noticed one was coming astray. Another time a jack socket in a vintage bass failed on me. And another time, a jack socket on a modern bass failed on me. And then there are batteries (one of the reasons I tend to avoid actives these days).
Still, I don’t always take a backup because sometimes it’s not practical (I don’t drive, for a start).