I have about 15 of the darn things...including one from my last trip, two weeks ago, that is the new Year of a Million Dreams design. Yes, there is signage in the area where you buy the mugs that states the mugs are to be used for the lenght of the stay in which they were purchased. There is also wording to that effect on the mug itself.
Do people bring them back? Yes. I have seen many different designs at the food court drink stations. The only time I really get in a snit? When someone brings their McDonald's plastic big gulp cup up and fills that up!! At least if you're reusing your previous mugs, Disney has gotten a little something from you.
Do I bring my mugs back? I did one time. I had a POP mug, and was returning for one night/two days. I knew I wasn't going to be drinking a lot..just a soda or two and one coffee. So, yes, I brought my mug back. Does that make it right? No, absolutely not. Do I care what others are doing? No, absolutely not. If you ask the CM's about reusing the mugs, they will give you the answer you want to hear. They will also seldom say anything to a guest who is not using the 'correct' mug at the drink station.
We all know the rules, they are stated in many places. We all need to do what we think is best.