Well, knowing that the service we speak about is paid for by tips then I think it's encumbent upon us to tip. We will pay their wages in some form or another.
One cruise I was on Mrs. Thomas dropped me off at the baggage pick-up area and drove the car to the parking lot. The porter took our luggage out of the trunk and put it on his cart as she was driving away. I reached in my pocket and all I had was $3 as she had our cash with her. I gave him the $3 and walked to the terminal to meet her.
One of my bags had some mesh elastic sleeves on the outside. I had a very nice pair of sandals in those sleeves. When I got my luggage on the ship I only had 1 sandal. I checked at lost-and-found throughout the cruise, no sandal.
Now, if he had wanted a nice pair of sandals (steal) he would've taken both of them. But he wanted to penalize me so he took only one. What lesson is obvious? When dealing with people who you are depending upon to give you service it's best to treat them as they expect. It's what they can do that you want to avoid.