Spare shutters seems to be more important than specifically spare cameras. I'm strongly considering picking up duplicates of the lenses for my Mamiya TLR lenses before I pick up a second 6x6 body, but that will all depend on budgets and what I find available at the time.
Then there is also an issue of mindset - I don't own spare cameras, but I do have multiple cameras of the same model. I bought a second 7D years ago after I thought about how much I was spending to travel to events a few times a year, and being able to continue to get some photos out of an away weekend became kind of important. It would really suck to book time off work, travel, hotel, and pay for all my meals on a long weekend just to get there and hear a weird clunk or something on the first shot of the first game. Plus, as a mainly prime shooter it meant that I could have two focal lengths ready to go at any time.
I'm very much against having 'a spare' camera that sits on a shelf or in the bottom of a gear bag till it is needed, and very big on actively using multiple cameras. It is a little less of a deal with film cameras, as there are less options to fiddle with and configure, but it would still rather suck to pull out a camera 'in an emergency' and surprise! something has dried out/gummed up/or jammed. By having two cameras in a primary role, such that they're both actively used on a regular basis, then you're more likely to notice any developing issue with them.
And in the digital world it can become even more important to use the two cameras regularly, and ideally have them of the same model. - What kind of settings and changes have you made to the main camera you work with? Pulling out your 'backup' camera when you're already frustrated over the malfunction of your main camera is not the best time to remember that you've since changed some critical setting or reconfigured some functionality. Surprise that button actually does something else!
Plus have you charged the battery lately?
It really comes down to "What are you doing, and how boned are you if a camera breaks?". I'll likely end up with multiple large format cameras over the next few decades, but I doubt I'll ever treat any as 'spares' to each other, and be unlikely to lug two along on an single outing.