That makes sense if you're paying per class or for up to x number of classes per week or month. Or if it includes private lessons. Doesn't make sense if you pay a monthly fee for unlimited classes.
My BJJ school has the option where you can pay to come in twice a week, you can pay to come in three times a week, or you can pay for unlimited classes per week. Of course its not truly unlimited as you are limited by how many classes they run per week. Still if you want to come in more often I think its a good option, its only about a $10 dollar difference between unlimited and three times a week and only about a $10 difference between three times a week and twice a week.