I knew a star football UI player (All-Big Ten) who ignored classes, blew his NFL signing bonus, failed to make the team in preseason, and I'm sure wished he had a solid degree to fall back on. I ran into him visiting on campus the following fall and it was clear that he was in shock and sensed he was screwed. He had a nice sports car to show for it, however.
Ex ante, taking classes seriously if you're an athlete is simply common sense. Few make it to the pro level; many of those who do don't last long and lack skills transferrable to the labor market in their mid- to late-20s.