In students' junior high and high school years, they are flooded with these semi-arbitrary quantitative observations of their work and abilities. Once they leave high school, some go on to college where they get more grades, many of which are even more arbitrary than any they have previously encountered. The activists are evenly distributed between the two sides of the issue, and both sides have good points.
Grades have their advantages and disadvantages.
Those who propose doing away entirely with any sort of evaluation in schools are insane according to traditional educators, the old guard says education without grades is meaningless. They have different issues. It's not necessarily true that some subjects are better suited for numerical grades than others. For most people, math comes to mind, but that's really quite debatable. The reality of it is that very few things we teach in school's these days are well-suited to numerical grades.