[Diesel_Dave]

There's some value in both Cd and Cd*A. Cd*A is what does ultimately determine the aerodynamic drag force, however, Cd allows comparison of different sized vehicles. For example, it makes no sense whatsoever to compare the Cd*A of a tractor-trailer to that of a small passenger car, however, comparing Cd is a fair comparison.

It also makes sense for scale testing in wind tunnels. Cd stays the same for the scale model and the full-sized version, Cd*A obviously changes (because of A).