As I've stated before on this topic, it is a custom as long as people play. Once you stop playing it is no longer a custom.
It doesn't seem like a hard choice to me if paying a going rate of pay is $140 per cabin per week. This added to the cruise fare makes it a lot easier to justify a tip to someone who is truly outstanding.
The one thing I will never understand is why you should tip at a restaurant based on the amount you spend. It takes the same effort to deliver a grilled cheese sandwich as it does a filet mignon.