I agree, but only if you tried really hard. I believe that the main benefit of exercises is not so much getting the right answer (who cares anyway?), but the fact that they made you go over and over again over the concepts, forcing you to really understand, to reread your notes, and to apply them, even if you fail at first. If you do this, it will stick, and, as you said, the next time you come across said concept you will be much more likely to get the subtleties and, suddenly, that mother of a problem becomes trivial.