They might be learning more from 'optional' homework though. For instance, my dynamics professor didn't issue any graded homework, although the homework in the class was my largest time sink last semester. The homework was scored so you knew what you did wrong, but the entire course was graded on tests (which were difficult enough that afaik nobody managed >70% before the curve). After the first test, it was pretty clear that the homework wasn't 'optional'. While this did help w/ teaching self-study, overall I think it was a poor way to run the course, since then the tests became overwhelmingly stressful for nearly everyone. I think that grading is necessary, and at the college level quizzes are typically pointless, but without homework there is little left to judge comprehension on.