Obviously I aim to pass these courses, and as many people have pointed out, the quizzes are a way of learning in themselves. In order to get as high a mark as possible it's often necessary to watch the video lectures more than once, and in some cases to do further research. In one or two courses the answer simply isn't in the lectures, and I think this is quite deliberate.
What I have decided for myself is not to repeat any of the quizzes unless there was a technical issue (i.e. an error caught after I had taken it). My first mark, is my mark, and if it's less than 100%, I take note of what the correct answer should be and move on. That is how testing is done in the real world, and as I'm not here to gain entry to or win anything, if I re-take a test, all I'm doing is trying to impress myself with a higher score. That's pointless to me.
I am failing one course. I may take it again next time it is offered. But even if I don't, I have learned so much from it that my time has been well spent. I understand the concepts, I just struggle with the math, and the score I get reflects that aspect of it. I get the conceptual questions correct.
When it comes to peer marking of essays, the hardest part is the marking. My son tells me this is commonly done in school these days, and I hope they are given plenty of guidance, because frankly I agonize over it. In the last essay I marked I found myself obliged to give 6/7 according to the rubric, of two very dull essays that didn't really understand the purpose of the assignment. While an excellent essay could only gain 7/7, despite being exponentially better.
But I see no alternative when there are 50,000+ students in some of these courses.