Good idea actually. How on earth are people supposed to get caught speeding if there car is so dirty the plate is unreadable!?
On a serious note, its a good idea. A very dirty car can have serious issues with viability, especially if mud or dirt is partially blocking head lights or brake lights.
Lucky I always brake when I see cars a further up then me start braking. Had quite a few filthy cars where their rear lights are so dirty you cannot see the brakes come on!