I prefer the drive fast.. take chances method....
OK.. a bit more seriously, I typically send out space parts for things I feel are most likely to fail. It also depends on where the show is taking place and how hard it would be to get them new parts in a hurry and how much they are paying. It's unfortunate but true (at least where I come from) but smaller theatres with little to no money just don't have back up systems. If the light board dies, they can either cancel the show, or do the show in work lights. So I try to have conversations with the powers that be about expectations when time allows and hopefully prior to the proverbial hitting the fan. I do however make one thing perfectly clear to all artisitic leadership.... there is nothing you can do, no money you can spend that will 100% guarantee that you'll never have to cancel a show. (even if you take human illness out of the equation) As soon as you think you have a spare of everything you might need... something new will break you've never seen before.
Case in point... at one theatre recently we thought we had a Nexo amp failure. It was popping a breaker. Turned out it wasn't the amp... it was the power sequencing bar built into the rack. Who would have thought of that? And if the entire thing had failed rather than one outlet... we would have been screwed... pretty much no show until it was fixed.
Anyway, a good example I use is when I've sent SAC out to Idaho since that is very far from home and although Boise has most of your typical city amenities it takes at least 2 days to get things there unless you blow the wad on over night shipping. So I sent out a 2nd tower with sound card and spare MOTU I/O Modules and spare preamps. They have plenty of the typical computer parts so no need for that stuff. We don't have spare for everything, but for most of the stuff that would typically fail.
I would do something similar if I was doing the set it up and tear it down every night thing as well. And it's what I do for my local one off events or important recording gigs, etc... have an extra computer and all it's cables, have extra I/O capacity and extra convertors. That should cover 90% or more of issues involving the SAC system.
Carrying around an analog console or even another small digital desk seems odd to me. Yeah you've got the "I can grab it and plug it in no time flat" thing going for you, however it's going to be a completely different architecture of console you're dealing with, you likely won't have enough inputs and if it's analog you'll have little processing unless you're hauling around an outboard rack as well. There are probably scenarios where that might make the most sense, but I don't think that would fit the bill very often. Unless the SAC system was just dead out of the truck, you don't typically have time to completely rethink how you're going to mix the show to the degree this would require. Of course if the option is that or no performance, maybe that is acceptable. But it would rarely work for me.