Going to class 3x per year: extremely low cost. Lost of free time. 2.6 GPA. Going to class every day: extremely high cost. Little free time. 3.x GPA (typically).
Given that semesters typically take 100+ days, you're talking a 30x increase in time spent to get a single point GPA improvement. Lame. This shows a problem with the classes and the teacher, as the teacher is apparently useless, not the student who is clearly smart enough to nearly BS a class and still get an easily passing grade.
Personally, I tend to fluctuate between 3.0 and 3.5. I do better in a lot of classes when I don't kill my brain listening to a bad teacher, and instead just read the darned book, which they test out of anyway. If going to the class is useless, and actively harmful, and you can get a good grade without going, what logic says you should go?
GPA is a laughable measurement of capability. It's much more of a measurement of boredom / lack of social skills. All my programming profs will (and have) happily write me a glowing recommendation, despite having a C average with one, and a low B with another. Of course, the C average one teaches level 450+ classes, and typically fails 50% of a class or more, so a C isn't too bad anyway.