I have never broken a string or had a bass fail and I don't have battery problems since I started changing them at regular intervals, so I don't use a backup bass as such. I really don't see the value in a cheap bass to sit around for "just in case".
I have several go-to basses, a P and a couple of J's, so I may take 2 to a gig or I may not. I might try a couple of basses during the sound check and then decide which one to do the gig with. A couple of times I did the first set with one bass and the second set with the other, just to give them both a run out but I rarely switch after the gig has started.