I don't understand your point. If a genius takes the class, then they'll get a good grade in any grading system. I'm talking about a student who starts off poor

I don't understand your point. If a genius takes the class, then they'll get a good grade in any grading system. I'm talking about a student who starts off poorly and does badly on some early assignments, then improves dramatically during the course. Even if they get to the same level of knowledge as the genius, under the traditional grading system, the early poor grades will stand. That will influence their final average/GPA even though those early grades are no longer relevant.