An 'A La Carte' menu of courses is an interesting concept. However, I think if everyone had the choice to pick any courses they wanted to, then they would naturally trend towards picking the easier/fun courses, and not the tougher ones. This would lead to people graduating without doing the hard stuff.
Sure, some students that are disciplined will take the proper courses that are hard (maths, etc) that have hard exams, teachers they may not like, but want to learn the material. But my guess is not many would, it's sad but true.
Imagine the same concept for high school. If we had the choice to take whatever we wanted in high school I have a feeling that 'Gym class' would be overbooked past capacity lol, while math and english class would be cancelled for not enough students
One point you bring up though is the idea of people taking classes just for the sake of having to fill the units up. That does happen. It would be interesting for the university to not measure things in 'years' but just let you stay as long as you want/need to stay a student to complete the courses required. So if a class is full one year (or not offered) and you really want to take it, you can just hold out on that class and take it the next year or another year when it opens and have an uneven course/unit load that year. I suppose it's possible, you could just stretch out your years and stay longer taking less classes, but then you wouldn't count as a full-time student.