There is no rational argument for grading people on a class curve
It's an imperfect solution for a legitimate problem.
The counterargument is that it would be rare for a prof/teacher to absolutely nail the correct difficulty for an exam.
If they feel that their students' average exam grade fails to accurately reflect how well the class is learning the material, then some kind of adjustment makes sense... imagine a scenario where you have a classroom full of motivated and engaged students who are showing good understanding of the material, and yet the average exam score is 60%. This strongly suggests professor/teacher has erred and an adjustment is in order so that the scores better reflect actual mastery of the material.
There are of course also a lot of situations where grading on a curve is blatantly unfair and/or simply makes no sense.
This is why I'm generally against grades with more than three levels.
I would like to see a system where almost all (something like 99 %?) of students just plain "pass". Then we have the lower statistical anomalies (lowest 0.5 %) and upper statistical anomalies (highest 0.5 %) which fail and get an "excellence" type grade.
If the average score is too low or too high, that doesn't mean more students fail or perform excellently, that just means the teacher needs to recalibrate.
(Of course, these percentiles ought to be measured over as inclusive a reference class as practical. Ideally across schools and years simultaneously.)
The counterargument is that it would be rare for a prof/teacher to absolutely nail the correct difficulty for an exam.
If they feel that their students' average exam grade fails to accurately reflect how well the class is learning the material, then some kind of adjustment makes sense... imagine a scenario where you have a classroom full of motivated and engaged students who are showing good understanding of the material, and yet the average exam score is 60%. This strongly suggests professor/teacher has erred and an adjustment is in order so that the scores better reflect actual mastery of the material.
There are of course also a lot of situations where grading on a curve is blatantly unfair and/or simply makes no sense.