I think that you have answered your own question. No coffee or tea; then why do you even need a mug?
SS Norway (12 times) '85 - '97; Radiance OS 06/01; Island Princess 11/04, Grand Princess 11/04/05, Golden Princess 7/21/06, Golden Princess 2/24/07, Explorer of the Seas 12/23/07, Island Princess 11/15/08
since you are not going to drink the coffee or tea, the only reason you might want to bring one would be to keep your water or soda chilled. We have not felt the need to bring one on any of our previous cruises.