I like carrying my coffee around with me at the parks in the morning, so I bring my mug. This is really easy because we still have a stroller for our youngest, so I just wash out the mug and throw it in the stroller basket.
I also find my mug handy during the day, because the sodas you buy in the park are huge (in our opinion!) So if we eat lunch as a CS place, I always have soda left over or I'm sharing with one of our kids. So I use the mug to store the soda we purchase in the parks.
At the end of the day, it's nice to have the mug handy at the resort on the walk back into the room so we can stop by an refill.
When we don't have the stroller along for storage anymore, not sure what I will do (with the mug, sweatshirts for 3 kids and 2 adults, etc!)