Question about Max-All-Session vs. Expiration attributes

I have a successful wifi captured portal system running with FreeRadius and HP Procurve equipment. When I originally started learning how to build it, I used WiFiGator as my first test case. When they set up that system, they used both the Max-All-Session and the Expiration attributes for all users, so I am still doing this now. We sell our time in Day, Week and Month increments, and the users are free to used the system as much as they want during their time. My Question is, do I really need to use Max-All-Session if all I really need is a hard expiration date for my users? Thank you in advance.