I would take the ID in case you want a refreshing beverage, if you look younger than about 40. We have found Disney to be pretty diligent about carding if there’s even the remotest chance you could be under 21. I have only had to use an ID once for actual admission, when there were some technical issues at the gate, but that could have been because I have an AP so I needed to prove I was the person named on the AP card.