I was reading a different thread, maybe on the NCL board, that some prefer to pack their own to use on shore excursions. Some comments said the supplied pool towels were too bulky, or even hard to get on shore days - required going up to the pool deck to get before going on shore, others said they are usually available to sign out at the exit.
Personally, I don't think I would want to both packing my own, plus I prefer a clean towel for the next use instead of reusing mine, or trying to wash it out every day.