The point that I am trying to get across is would you rather walk to the bar and get a drink and then pay the 15% gratuity or have it available in your room? I guess the other option is if you want a drink in your room you can call for room service and then you can feel that you are getting your 15% worth.
Your reference to tipping the cleaners or engine crew members is a little of base because they are paid competive wages and do not have to depend on tips as do the service employees. I am sure that most service employees would rather have the tips instead of a set wage because they know that if they give good service they will be rewarded more for their efforts. The set gratuity is just to make sure that they do get minimum. As for tipping more there is nothing stopping you from giving anyone a little extra. I am sure that they appreciate it.