Whenever we eat at the resort (usually at least once per day), we get the big bottles of water for our drink with ddp, and take those to the park that day. We use the refill mugs for drinks while we're eating at the resort.
If you are at one of the resorts with a frozen coke machine, the mugs are a must! (frozen coke in the morning on the way to a park, frozen coke at night on the way back to the room).
to expand on the above - the resort beverage stations are outside the serving/paying area of the food court so you won't need to go through the cashier.
We did as others said and used the mug at breakfast and brought back the juice/milk for later use. Actually I would go by myself to breakfast. I'd get my meal with a bottled juice or milk, get ice drink my dr. pepper that I brought with me over the ice in my mug, rinse the mug and refill it with coke. I'd then take that coke and the juice back to the room for the rest of the family to have drinks for their inroom breakfast of granola/oatmeal/cereal.
We looked at the mug as a souvenier. Sure we were going to use it some but didn't stress over getting our 'money's worth' out of it. I probably did fill it 5-6 times. But have filled it many many many times at home....all three of them.
If you are at one of the resorts with a frozen coke machine, the mugs are a must! (frozen coke in the morning on the way to a park, frozen coke at night on the way back to the room).