Reason 1.
I am tired of overhearing statements like:
"I don't like the teacher, but I love the worksheets."
"That class is an easy A."
Reason 2.
A grade can (huge emphasis on "can") show how much of a curriculum you've mastered. But what happens when you get rid of the curriculum?
Reason 3.
Grades have become all school is about. If we removed the current idea of a "grade," then we could get down to actually learning things.