At our recent trip to Blizzard Beach, the scanner was built into the drink dispenser. Scan it for a few seconds, and the dispenser was activited for plenty of time to get ice, fill up, etc. However, when you add a few seconds before and after (for the machine to reset) when there is a crowd, as there seems to always be at the AS resorts, the queues will grow and customer disatifaction will grow.
I understand if Disney wants to make more money by having the mugs only good for a visit. I think, though, that the current number of dispensers is not enough (and I don't know how they would modify the coffee urns), and they will lose a lot of goodwill if they do this. With the cost of goods (water, sweetener, flavoring) for a coke or cup of coffee being literally a few cents, they already make a large profit selling a plastic (25 cents production cost?) mug for ten bucks. It gives the guest the feeling they have a "good deal" (we always fill up our mugs a lot, but wouldn't pay individually for that many drinks, so the savings are mostly illusory).
I think their proposal will lose more in goodwill and opportunity sales (buying food products) than it will make in profit, though perhaps they've considered this and conclude otherwise. However, above all, trust is the most important aspect of this - for those of use who were told by CMs that the mugs did not expire (good for a lifetime), it's easy: Disney should honor that promise they made, and design new mugs differently when they are sold with different conditions. What if I purchased an Annual Pass, good for a year, and three months into my ownership Disney decides to charge me every time I visit their parks?