From what I understand on the mug issue, a mug purchased at XYZ resort can be used endlessly at that resort. I have in fact taken DL mugs to POR and had it be fine - in fact the CM's were tickled that I had the older mug and happy to see a return visitor.
Now, here is where the real discussion begins - using XYZ mug at ABC resort. PC or not, I have used that same DL mug at two other resorts with no problems. I figure I spend a whole bunch of money there while I visit, and I visit often. AND - I don't abuse it - really, I drink mostly water anyway.
I have also taken a rubbermaid sippy and filled it with ice water from the vending before heading to the parks. Never any problems.
Good luck!