...at what point do you disengage that thinking and why?
Ex: extra strings, extra cables, fuses etc.all make sense to anyone and if you're on the road a second bass is of course a good thing but.
Where do you draw the line and why is that different than a backup bass?
I do a cost/benefit (risk) analysis, although it's no where as professional as it could be. The thinking goes like this:
A. Can I do my job if [insert piece of gear, piece of clothing, name of banjo player, etc.] terminally malfunctions mid-show?
B. What are the odds it will terminally malfunction mid-show? (Nod to @JIO for the critically important maintenance point)
C. What is the cost of mitigating the risk?
Let's say my bass gives out mid-show. At a given venue, the real risk might be a delay of two hours while I go get another. In said venue, the delay might cost us the gig AND future bookings. Not to mention fan goodwill...
Cost of carrying a second electric bass? First, there IS the cost to obtain and maintain the bass. (But I already own enough of them)
If I have the space, there's 10 extra pounds my car has to carry (small, technically measurable gas cost). Space in said vehicle. Will that cause another band member to have to drive? Cost of that gas?
(You can carry that line of thinking as far as you'd like. Risk of car catching fire going down the interestate, loss of both basses? Infinitesimal, but sure, there's a risk associated with carrying two.)
I can't afford two sets of PA speakers or an understudy for the singer--not to mention the cost to haul/manage those items--so I have to accept the risk that there might be a malfunction there and mitigate some other way.
- There are two speakers. The band can perform with one OR unplugged, if necessary (we're mic'd acoustic).
- Everyone in the band sings enough leads that each of us can carry a full set if lead singer chokes.
I'll stop blathering now. But if I am good at maintaining my gear, the risks are lower and more clearly known. If I'm not sure, I decide what the cost of the backup is. When we toured four states away, I carried a backup power amp, microphone, and two-of-everything cables and cords.
Disclaimer: I don't actually carry two uprights to my bluegrass/acoustic gigs--don't own two, wouldn't anyway. Cost to transport more than the cost of the risk of failure. Most gigs we do could still be knocked down fine if a single member, voice, or instrument craps out.
Caveat: For weddings I always take extra precautions, because I care about not blowing those more than most other gigs.