I think this would actually help out the whole meta-game that is free roam in a big way.
You will have to save up more money to really get the most out of owning a car. Want a million dollar super car? You will need to pay an extra 3 million for insurance, in total it will cost 4 million.
This sorta gives the cars more value, people don't really have much to save up for so why not start like this. Now you can always buy the car without insurance if you are impatient and you could safely select it in races no problem.
However if you take it to freeroam, you will find yourself taking way better care of it and player interactions will now be more intense. This is a good thing, you just don't think so right now because you are f*cked in the head for thinking otherwise. I know my rights.
Destroying a guys adder and discovering he didn't have insurance after he tried to run you over would be so damn satisfying. It would be like, yeah bitch, its f*cking gone now wtf are you going to do. You would be sent to bad sport, and it would friggen mean something.
Destroying a guys adder and having to pay like a million dollars for it, essentially bankrupting you, would make you feel like a douchebag and the owner would get off on it. If he had no money then the insurance company should pay for it. Also tank owners should have to pay insurance I really don't know wtf R* was thinking there.
Moreover it would be cool to see people driving expensive cars because you know they really worked for it since they are so damn expensive. It would give you something to work for etc etc.