OK, here's a scenario.

You're hosting an event. You want to provide the attendees (~2000 people) with water. Do you provide large water carafes with spigots, and disposable (4-10oz) cups? Or do you provide (20-30oz) water bottles?

In my experience, people (especially college students, since that's the demographic I'm used to working with) are much more likely to hold onto a water bottle until they finish all 20oz or so, but grab a cup, fill it, drain it, and throw it away...then go back 20 minutes later and repeat the process.