We always try to stay at different resorts, and I like collecting all the different mugs--they're kind of neat souveniers; better than t-shirts that fade and wear. Of course, we don't buy them for the kids. I don't want them to have too much soda and they don't like tea and, of course, no coffee.
Remember, the point of the mug is to give you a price break. So, if too many people are using the same mugs over and over and over, then WDW looses money, and the cost of the mugs either goes up or will be done away with completely. Before buying them, you do have to decide how often you'll use them. DH is a big tea drinker, not something we can stock up in the room. I like my coffee, but I'm not going to brew it 4 times a day.