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.