I'm sure that other benefits could be imagined as well. "Poor people" are probably much more likely to own high-mpg 'clunkers', or gross polluters, e.g., which would partly justify the program on environmental grounds.
I assume that you meant "Low MPG clunkers" or "High mileage polluters." When the cash For Clunkers program was announced, I was skeptical, but it really did get a *lot* of terrible old gas guzzlers and oil burners off the road. They were largely replaced with Hyundais, which probably wasn't intended, but a few Fords and Hondas were added as well. In balance, it both helped the working poor and helped the environment.