What a load of interesting information! I really like these facts and figures, especially things related to administration.
Emsr2d2, you say "Keeping the registration document in the car is an extremely bad idea though (and I know a few people who do this). If your car is stolen with the document inside, then the thief has the car and an ownership document."
Here the registration document says who the owner is. There's your name etc. and that's the point, the police don't need to contact anyone, they can see immediately if the vehicle belongs to you or not.
Let me pop a similar question.

How about health insurance? Do you have to carry any cards with you? If you have an accident or if you need to see a doctor, do you need to produce it?