While technically true, I was not comforted by the thought of driving over a bridge or riding in an elevator or flying in a plane that had been designed, constructed, and built by an engineer who earned D's throughout her/his college career. I would rather trust my life to the engineer who really knew and understood the concepts.
A wise man (Doug Reeves) once said: A "D" is a coward's "F." The student failed but you didn't have enough guts to tell him/her.
Grades, while they seem so clear, really don't tell us very much. I could earn an "A" in biology and still have failed a section or a unit over the course of a semester. Was that section or unit important? The grade of "A" would suggest that it was not.
But what if it was?
Students need to know what they know and what they don't know. Traditional grades are averages. And averages distort.
As educators we have an obligation to accurately report what our students know and don't know.
Traditional grades don't do that.