It's not that it has no marginal benefit, it's that it has diminishing marginal benefit for the effort spent. At some point, you're better off working on another class's assignment instead. At some point you're better off taking leisure time, or even sleeping. If even people with a good understanding are only expected to get 50% correct, how are you supposed to know when you're better off going to sleep, only knowing that you've completed 50% and not knowing whether the extra 5% from sacrificing some sleep is worth it?
The returns diminish when it comes to impact on your grade, yes, and I certainly enjoyed transparency about how the grades I got would be impacted by my work.
The distribution of value for learning, though, goes up with difficulty until it drops to zero (the point at which you cannot solve the puzzle at all). My only point is that we should strongly prefer systems that allow us to soak up all that high-intensity high-value work -- modern universities aren't that for many students, though, but independently reading textbooks could/should be.
Your job, should you choose to accept it, is to comment on this thread explaining the most awesome thing you've done this month. You may be as blatantly proud of yourself as you feel. You may unabashedly consider yourself the coolest freaking person ever because of that awesome thing you're dying to tell everyone about. This is the place to do just that.
Remember, however, that this isn't any kind of progress thread. Nor is it any kind of proposal thread. This thread is solely for people to talk about the awesome things they have done. Not "will do". Not "are working on". Have already done. This is to cultivate an environment of object level productivity rather than meta-productivity methods.
So, what's the coolest thing you've done this month?