> Why should we own cars?

In the city, I couldn't possibly agree with this more. When I lived in Boston I sold my car after the first two years because it was a ridiculous expense and hassle for the utility. Taking a cab a few times a week turned out to be cheaper than parking tickets alone.

Given a choice though, I much prefer to live in less densely populated areas (e.g., northwest Montana), and owning a vehicle is really unavoidable. In these places that also means a 4x4 SUV or truck, not a Prius, or else you'd be calling a tow truck every 5 minutes in the winter.

In order to control what happens in the car when you're not in it.

I would expect driverless taxis to end up like public toilets.

You don't have to register at the toilet with your ID/driver's license. You do however, have to do that when you use e.g car sharing, and the first question the system usually asks you, is: "is the car clean?". If no, then the previous renter will be liable. So this is already a solved problem.

If public toilets had a user reputation system (like Uber), they would be as lovely as the ones in Japan. Unless you had a bad reputation, in which case you deserve what you get.