You need to carry a backup bass with you for gigs if you're getting paid. The show must go on no matter what happens. So that means you need at least two basses.
Nowadays I can't see gigging without two or three, due to rounds/flats/tunings requirements.
But if I did not (like back in simpler times when a set of RS66 tuned to EADG were all it took to get through a gig) then I'm not sure I agree with that statement - because I haven't broken a bass string in 25 years. And the last time I did break one (an A string, as I recall...) I finished the song by holding the broken string up off the others (the core had broken at the saddle, but was still attached by a winding). When the song was over I snipped the string off, and finished the set with three strings. Always keep a new spare set in the case, and also a backup previously used set, usually...