John, there are better ways to assess a student’s learning than with letter grades. A well-designed assessment can reveal to a teacher exactly what a student knows and can do, as well as what the student is ready to tackle next, allowing the teacher to target his or her instruction appropriately. Not every student in the room is ready to learn the same thing in the same way at the same time at the same pace. Old-school grading like you described encourages a “teach to the middle” attitude. Students “earning” an A are rarely challenged, and students “earning” an F are generally lost from day one.
Letter grades are a relic of the early last century; they became outmoded as soon as we developed technology to track student progress in more effective ways.