I agree with you Georgie. We always leave extra tips because we get extra good service - never had bad service on a cruise - and we feel good about doing it. Leaving extra tips means that you have really enjoyed the cruise.
We always treat the service folks with respect - maybe that is why we get good service.
ps-- I leave hotel maids tips too, and some people think I am nuts. Maybe, but I would feel guilty if I did not leave a tip for good service.