Financially, having a second vehicle is almost always a dumb, unnecessary luxury. As others have noted, there are lots of ongoing costs even if you don't use it. If you choose to keep it, recognize it as the luxury it is - we all have some of them, but don't fool yourself into thinking that it's anything but.
I don't have even a single vehicle for myself, so the idea of a spare, just in case, is on the other side of the spectrum. We have one for the family (which I do drive the family around in when we're traveling together), but all personal transport and errands are by e-bike. Though our car is getting a bit older now, nearing its tenth birthday, the number of times we've required that spare vehicle "just in case" over the 4 years we've been a one-car family has been: zero. We were 'car-lite' for two years before that, where the spare car was almost never driven. We did have one "just in case" incident during that time when the primary car's battery died. Not having the second car would've been a bit inconvenient at that point, but we certainly could've overcome that for a minimal cost and better preparation. Nowadays, we sometimes have to coordinate things to make it work for four people with one car, and occasionally have to sacrifice here and there. Nevertheless, it's been totally worth it. I can't imagine personally justifying the waste (cost, storage space, aesthetics/ugliness) of having more cars than we have drivers.