We all have to make our own choices. For me, backup is well worth the minimal effort for the following reasons (and probably more if I gave it more thought) -- not necessarily in order of importance:
- IMO, it would be inexcusable not to have backup in the event of bass failure at the gig. Every guitarist I perform with brings 2 or 3 guitars; no reason for a bassist not to bring at least a 2nd even if it won't be played
- Backup is like insurance: provides peace of mind
- I like having the option of choosing between basses at the gig, and I sometimes play more than one. So it's for fun as well as prudence
- My backup bass also provides backup strap, backup cable, backup clip-on tuner, possibly backup strings and battery
- Doesn't apply to me, but some bring >1 bass so they can do fretted and fretless, 4 vs 5 strings, etc: that's built-in backup
Finally: gigging is risky, but that applies to the primary as well as the backup. I usually keep both basses setup and available onstage for easy access, which equalizes the risk. And given that we're talking actuarial analysis: as noted I've had some bass failures in those thousands of gigs, but never had one disappear, never had one damaged by a drunk customer or bandmate, etc etc. Theft/damage could happen to me this weekend, who knows -- point is, I haven't experienced that, while I have experienced a half-dozen or so failures.