In the 'old' days, each resort has a resort specific mug...it was nice, we bought one on each trip and now have a nice collection of about 18 of them. I have three mugs from DxL (now POR) that were sold as 'good at DxL forever'. We returned to POR 18 months later, but did not take our mugs with us. Bought new ones. Those mugs were good at the resort they were purchased at, for the stay in which they were purchased.
Fast forward about 8 years. Now, we have a ton of these things...from CR, Polynesian, BCV, BWV, ASMusic, POP, etc.
Disney stopped putting different resort designs on the mugs....they became a generic YOMD design. Then, they started selling them as...good at our resort beverage stations during the stay in which they were puchased. That made it nice if you had a split stay...you could buy a mug at the first resort and then take it with you to the next resort you were at.
Do guests reuse their mugs, on subsequent visits? Sure they do. It does make it hard to enforce the 'stay in which it was purchased' ruling when all mugs from the past 2 years look exactly the same. Does Disney want you to keep those mugs and bring them back with you next year? Probably not. Do I think the CMs are going to do anything about you reusing a mug? Nope. Especially if it's exactly the same mug they are selling now.
I've seen people refilling mugs from other resorts, old mugs from the resort they are at now, McDonald's huge drink containers.....people will belly up to the beverage stations and refill just about anything. If you are bringing back your Disney mug, at least Disney has gotten some return on it!!!
You are probably going to get a ton of mixed responses to this question. There are those who feel that you are never, ever to resuse a mug...you must buy one each and every time you go to WDW. Others will tell you it's okay if you use a resort specific mug at that particular resort.......WL mug (no matter how old) at WL. Still others will tell you it's okay to use your DxL mug at the GF.
And others will say it's okay to bring your Wonder bread bag, emptied of the bread, and fill that up!!
Do what you feel comfortable doing. I'm not saying what's right or what's wrong. Each person will make their own choices as to what they feel is right or wrong.