I have mugs from all our visits. If it's a new resort we've never stayed in I'll buy a mug. Not only are they useful to us, but they make great souvenirs.
That said, this trip we stayed at WL and POFQ. I bought a mug for WL, but had a mug from previous trip to POR. When we moved from WL to POFQ, I used both my WL mug and my old POR mug at POFQ. No one, not even the CMs batted an eye.
During that stay, I saw several people using BB mugs, old Dixie Landings mug, old POFQ mugs and no one batted an eye at them either.
If I'm going to Disney, they'll get plenty of my money besides the refillable mugs, especially since I only fill mine with water. The price of soda isn't going to break Disney's bank considering their deal with Coke. Besides, how are they going to know if I bought my mugs now or last year since the design is the same?
If anyone wants to flame me, be my guest.