Every time I go to WDW I buy the Mug. I figured out a way to make it worth it. If you are going to pay $17 for the mug you should be able to make it worth it.
Here is how you can make the mug worth the money you pay. All you need is a wide mouth bottle.
First thing you do is fill the bottle with ice.
Second fill the mug with your drink.
Third pour the drink in the bottle.
This is all legal. You are using the mug for the drink. All you are pouring it into something else.
Doing this you can have something to drink when you go to the park.
I carry two bottles so I will have something to drink while I'm in the park. To do this all you have to do is wait for the time expire from the first fill.
After those bottles are empty I will go to a Resort's food court and fill up the bottles. What I do is get something to eat and fill up the first bottle after I get the meal After when I finished fill the second bottle.
You can go to any resort to fill your mugs. You are not tied to the resort you are staying at.
By doing this every day I fill the bottle 5 times. To buy a drink in the park is $3. By using the mug I'm saving $15 a day. In a 7 day I'm saving $100 in drinks. So my mug is paid back within a day.
This is all legal. Disney does sell wide mouth bottle so what you pour in it is OK.