I shell out almost $1K a semester in student fees and (admittedly, heavily subsidized) health insurance. The savings of the previous semester are entirely wiped out in one check to the Bursar's office, as the university recoups some of the money spent on my paycheck. Working the system is just one way of saving money in the current economic situation.
The tuition and fees system seems completely incomprehensible. For tuition, you pay an extra dollar amount per credit up to 9 credits, at which point you can add another 9 for no additional charge. For fees, you pay an amount determined by some combination of the number of classes as well as the number of credit hours, up to 7 credits, at which point you can add another 12 for no extra charge. It makes far less sense than the abacus or slide rule to me.
But, there is an obvious way to make the system work for you, at least when it comes to flexible things like dissertation hours. Say you need to take 28 credits in four semesters. The balanced approach would be to take 7 in each semester, but this would be foolish. Then you pay maximum fees in four semesters. Instead, you can spread them out 6, 6, 6, and 10. Then you pay maximum fees in one semester only, with no extra charge for those last three credits, and save over $300. A small sum by some standards, but massive savings to the graduate student budget.
This is one of those examples of how procrastination can be useful. Think of the money I would have given the university if I had instead felt compelled to actually do the work my advisor had sent me.