I always carry my refillable mug with me to the parks. If I'm with spontaneous people or going solo, it has come in handy on an number of occasions.
For example, walking from Epcot to Hollywood Studios, and grabbing a beverage on the way.
After a long day at Magic Kingdom, stopping in at the Contemporary on the way to dinner at Disney Springs.
Getting a full mug of soda on the way out to the parks in the morning, which makes me not crave another soda at lunchtime.
Taking a mid-day, unplanned break to go resort hopping.
It would really be nice if there was an option to allow for refills at the parks. I realize that the price would go up, but I'm sure Disney has a good idea of what the average mug-toting resort guest spends on beverages in the parks and price accordingly. I guess new systems and equipment would make this more complicated... plus health codes and the lack of mug wash stations.