The annoyance with 10-20% means that in order to be an "A" student you have to do all the homework instead of just ace the exams which is obnoxious if you actually know the material. Edge case, I know, but that last 20% is a ton of extra work.
Realistically I think the more common case is to think you know the material, skip studying, and then faceplant on the test. Homework should help self-correct.
But yeah, I could it being annoying if you really do already know the material.