Cost reduction.
Sharing a bottomless soda between multiple people is strictly forbidden in most establishments. If you can all agree on flavors and dont mind getting each other sick, then you are set.

Road trip drink solution.
Smuggle several empty plastic gallon milk jugs into your booth. When clerks are not watching, dump your entire soda into the jugs. Refill. Remember a funnel. Getting them out is far more difficult than in.

Another idea is to ask for water (which is usually free). The cup given might be smaller, but you could go and refill it with soda in establishments where there is no supervision.

What amazes me about free refills is that people often still buy large or even extra large cups, yet you would have the same amount of soda (unlimited) with a small cup, and save a few dimes or quarters in the process. Of course, you might have to make an extra trip or two to the soda machine.