So - I haven't ever cruised but I have read a lot about them in considering a vacation, especially lately. And it seems like a soda drink package would be akin to what this person is asking if it's appropriate. While WDW doesn't offer a drink package and isn't all inclusive, I feel like a fair number of people are acting like this is completely unheard of and a sign of bad character. But obviously there's a model for it.
Coincidentally, I never separately get the mug (I have gotten it with the dining plan) because I'm rarely at the resort and feel like it's way too inconvenient to deal with (but if they had a parks-inclusive mug, even at a - obviously - higher price point - I'd buy that in an instant because I'm more likely to have a drink there). I also get really uncomfortable with getting a benefit that isn't, necessarily, obviously okay. For instance, if a server asks if I want my drink to go - I will almost never say yes because it feels like I shouldn't (even if I wouldn't care if others did, a server made the offer to me, and I would like it). I was also the kid who went to a friend's house every day for 4 years of HS and for those 4 years, my friend's mom would say, "Help yourself to anything in the fridge" and for the first 3.5 years, I'd still ask if I could get water or a drink with my meal, or anytime. It's just me.
So, if it were me, I wouldn't do it as I'd feel uncomfortable and care a lot about what other people think. If I saw another table doing this, I absolutely wouldn't care or make it my business. If I were a server and/or worked in food service (which I have been, in different settings), I'd absolutely want guests to do this to maximize their benefit/minimize some costs so they could spend that money in another way or simply enjoy themselves. If I were in some form of leadership*, I wouldn't care because to me the guest satisfaction would matter more than the 3 cents of cost (that said, this is why I've generally chosen to opt out of leadership in organizations where I would need to be more aligned with capitalism than customer-service).