Ever since I was stopped by the police because my "car" was seen near the site of a robbery, I keep both my plates on and check them all the time.
It wasn't my car but my front plate that was seen near the crime. Somebody stole my front plate and put it on the rear of their car. Fortunately their car was a Ford and I was driving a Porche 911 at the time - the cop stopped me just to let me know what happened. I went to the DMV that day and ordered personalized plates.
In San Francisco, they have really started cracking down on people without front license plates. Its easy money for the city. I got two tickets within 48 hours for not have my front plate on my previous car. The good news is, that if you get a ticket, you can get the plate installed and only have to pay $10 as a fix fee.
Hmm... maybe I won't put one on. My Cooper will be my first car, so I am not to familiar with laws such as this, all I know is that it's a law. I hate the front plate... I suppose now I'll just leave it off.