My partner and I have joint financial accounts, but it still would be weird to us to take whatever car.
Why? Is it weird to use whatever plate or fork? What about couch cushions? Umbrellas?
I see cars as tools, so there’s not “my car” and “their car,” but “the small car” (hybrid w/ great gas mileage) and “the big car” (minivan). Before we had kids, we only had one car, and if we needed to be in different places, I would ride my bike, walk, or take the bus. That almost never happened, and it wasn’t a big deal when it did.
Plates, forks, cushions, and umbrellas are not typically as personalized as a car.
And my argument is that cars should likewise not be personalized. They’re tools.