Princess Plus and Premier are money makers for Princess, or it wouldn't be offered. Just an example, if 2000 passengers purchased Princess Plus, that's $840,000 that Princess gets for the week. For a year per ship, it would be $43,680,000. Now multiply that by 16 ships. Yes, that money is distributed towards crew appreciation and Wi-Fi but it's still a money maker. It is also a great deal compared to Royal Caribbean that offers their drink package on sale for $82/day (give or take a few bucks) and it's just for drinks.
Have a great day and have a great cruise.