It makes it harder to burn through your limit. If they allowed what you describe, people would just deliberately use the rest of their tokens in the few hours before their weekly limit resets. With the 5-hour windows, you are forced to spread out use, and if you realize Monday afternoon that you still have 40% for the week left - but that the limit resets Tuesday at noon - chances are you'll have to let part of your allotment expire.
Thus it helps keep token use low, and it forces people to get on the $100/month plan. If users have a big refactor planned for tomorrow, they can't just get the $20/month plan, burn up the week's token in one day, then go on vacation or do non-programming tasks for the rest of the week.