Why even care about it being your car? How about just a car? At your bidding when you need it and no other time.
You might look at a car as a transportation vehicle, I look at it as an extension of my private space. It's my car, so I can customize it, I can make it comfortable for me, I can keep my stuff in it, I know it's always available, I know what state it is in, etc.
Regardless of where I live, I keep a toolkit, trauma kit, and emergency supplies in the trunk… just in case. I also will have region specific stuff:
- When I lived in the great white north, I used to keep a snow shovel, cat litter, etc. in my car during the winter months.
- While I live in an arid desert, I keep extra gallons of water and a solar blanket in the car.
I really like knowing exactly where my car (and by extension) stuff is. Also, I hate having to readjust my mirrors and seat.
Also dogs.
A manual lockout would be a smart thing here. There's all kinds of solutions to the "computers are dumb" problem.
I also think it's slightly less error prone, as there's no user input required, and since the stuff gets removed from the car, it's much less likely that anything will end up being sent to another user. At worst, they forget to put your stuff back automatically, and it's relatively easy for them to send another car, or even send your stuff to your house. Indeed, that might even be a service people are interested in if they're doing a lot of shopping, or they take public transport for part of their journey.