I think it all depends on your family. If you're big soda/coffee drinkers, then definitely yes! But if not . . . then it's a judgment call. Also depends on how much eating you do at your resort, and how many days you're going to stay.
I never got a mug on my previous trips, prior to last fall. I'm mostly a water/milk/juice drinker (also drink some soda, but no coffee or tea) -- so it seemed like a waste to get a mug. But I when I calculated how much it would cost to get a few sodas during my 10-day trip, it made sense to get a mug. I was really glad I did - it saved me lots of money.
It is a bit of a pain, though. I was at POR with a room that was a fairly long distance to the dining hall. I had to either: 1) rinse out and carry the mug with me to the parks in my backpack, to avoid the hike to and from my room to drop it off after breakfast; or 2) take the long hike back to my room, drop off the mug, then hike back to the bus stop in the mornings (then go back and get it at the end of the day to get more soda!). With all the walking at the parks, it seemed easier just to rinse out the mug and put it in my backpack.