You never get a second chance to make that first impression. What will your customers think if your agent rolls up in a filthy vehicle?
We are taught not to judge a book by it's cover, but this axiom is not always observed through the eyes of a customer. While a clean vehicle may not score you extra credit, a noticeably dirty one will definitely knock a few points off your first impression score. Why put your operators in a position where they start out with a negative image to overcome?
Another case can be made for maintaining the overall health of your fleet. A vehicle's finish hold up better when it is kept clean, and may lead to less exterior maintenance cost and potential higher resale value for that vehicle down the road.
Finally, a clean vehicle instills a sense of pride in the operator who drives it, and often leads to a double halo effect. Not only do employees strive to keep their assigned vehicle looking good, but they also rise to meet the image of their vehicle, which is a good thing for both the employee and the company they represent.