Back in my "Brit Pop" gigging days (when I was still a bass player), I would gig regularly with no spare - but having said that, I was meticulous in my preparation. I would check everything thoroughly before it went in its case and strings would be changed regularly and wiped after use. The way I thought of it was like a car - if you look after it and service it regularly, it shouldn't let you down, and in 3 years of almost constant rehearsing and gigging, my bass never once let me down. However, these days when I do the occasional gig (mostly school events), I would feel extremely exposed without a spare bass or guitar, spare strings, spare lead, spare batteries and a box full of spare picks, but I've still never so much as broken a bass or guitar string on stage in over 30 years of gigging - so I agree, it's probably as much an emotional safety net as anything else.