I support the general idea, but I have issues with some of the specifics... I think 5 mins between refills is too long, but I understand if they had completely unlimited refills it wouldn't stop people from filling their whole family's cups. I'll throw out another hypothetical: my family buys mugs for each person, but we don't always schlep around all four mugs. I guess that's my line... if my family has all purchased mugs, then I think we should be able to share between 1, 2 or 3 mugs for the four of us if we all drink directly from the Disney mug. Pouring from the mug into separate cups would be different, but the inconvenience of sharing the actual mug and the fact that we purchased mugs for everyone makes it OK to me.
One more nitpick: Disney will likely have to change the signs promoting the mugs. Before, while you were limited by the amount of time to fill the cup and the time to drink, refills really unlimited. Now, with four refills per hour, there is an actual finite number of refills you are allotted. My math puts the number somewhere around 1008 refills per mug. There are a lot of numbers higher than 1008.