You'd be surprised at the number of enthusiast's cars (hot rods, sports cars, and such) that are driving around unregistered with no plates in california, but with full comprehensive insurance coverage. California makes is exceedingly difficult to register cars with modified engines due to smog restrictions. Also, many people just plain think that they dont look good.
You almost never get pulled over in california solely for registration, and even if you do, if you get plates on the car and get it signed off, then you jsut pay a $10 fine and pull the plates back off the car. This is especially common with front plates, but happens sometimes with rear plates, too.