I am sure it's far too much effort to steal one of those cars, in the UK even un-tracked civilian cars aren't worth stealing these days unless they are worth a lot of money (to offset the cost and effort of changing the vehicle's identity). If you don't want to change the vehicle's identity you still have to ship them to places like Africa where they will sell for considerably less and you will have to pay for shipping (plus you risk getting caught at the shipping dock too).
Well you can still sell the parts, but are you sure cars are cheaper in Africa? In France it's very common that used cars of low value are bought by people who just export them to African countries (Ivory Coast, Madagascar, Cameroon...). Some friends who only recently came to France from Africa told me many cars can be sold 3 times as much over there.
But it may be nuanced a bit by the difference of price between cars in UK and France too, they're a bit more expensive in France. And I don't know how frequently they check cars when shipped, but I once used the ferry from Dover to France with a car, they didn't check anything, not the driver's licence, not any papers, not even took a quick look if something strange could be in the trunk. Just a piece of ID for the ticket as far as I remember.
Renault Twingo (a small quite shitty and cheap car for those of you who don't know it) was for years the most stolen car in France. I think recently the BMW X6 took it over. So I think it probably can still be worth stealing a cheap car.
But back to the "public" car topic, yeah I think it can be convenient for some people. We have it in some places in France, it's been around for a few years, but I don't know how it works (I think you need to pay for a membership) and I'm not interested in using them. They're a bit ugly but who cares, it's not bad that way because they attract a bit of attention and maybe encourage people to look into that alternative. And I think they're all 4 seats.
Here's a pic:
