First of all, I'm not one to have to have a perfect score on every paper. But...... if I don't get a 100 on it, then there is room for improvement. I understand that.
My problem lies in instructors who will give you a low A or a B and then give only praise for your work. I asked my instructor what I could have done better and her response was that I had an A- and shouldn't need to worry about doing better. My B paper add only wonderful comments on how well I did and how excellent the work was. Yet, there was nothing to direct me on how to make the B into an A.
Instructors should always give praise and constructive criticism. That is how you can learn and become a better student.