Sorry but I completely disagree. The D students I knew were dumb as rocks. I got As through my engineering degree with a few Bs in a few of the classes I was less interested in and I have to say, it's completely ridiculous to suggest the D students knew more than I did.

You could either solve the problems using techniques taught or you couldn't. Grades were a direct measurement of this ability. Maybe grades don't matter in Psychology or Sociology class, but I don't want to rely on an electrical system designed by the D students in my classes. It would probably kill me.