This is a very good question if it is about what there is to gain from a cabin steward. I agree with the previous comments that the "normal" room service is already about as good as I could want. In fact, in over 30 cruises I cannot think of a single negative related to a cabin steward. What to gain? Here are my guesses. Maybe a never empty ice bucket, quicker responses to requests, or more towel animals? 🙂 Beyond that, I honestly don't know. If we are talking about a bartender or dining steward, then I suppose there could be a bigger return.
Me, I would just lay that extra $ on them at the end as thanks for a great experience.