My wife is Filipina and we've talked about this extensively with Filipino crew members, who are usually the majority of the crew because they are English-speaking. What we learned is that most members of the crew earn about $400 a month from the ship. The tips supplement this and are collected by the cruise line and divided among the entire crew, including the folks in the engine room that you never see. These people work seven (7) days a week, sometimes on 12 hour shifts, unless they get a few hours off in port. They do this continuously for several months at a time, then they have to pay for their flights back home, a costly expense when you earn $400 a month. Yes, they earn more than they would at home but few of us would work under those conditions, let alone do it AND maintain a smile and good attitude. Hence we always tip the suggested amount and are grateful for the great service. You don't need to tip individually although none of the crew is going to say no.

BTW I don't like this system, it's not fair to either the passengers or crew, but it is what it is. Unless and until the cruise lines decide to change, I won't screw over some poor soul who works 12 hour days seven days a week.