I'm not familiar with Princess' drink package parameters as we usually sail with X and RCI. Personally, I would get a package if just for the convenience if I could get somewhat close to break even. But based on my habits, I find the math pretty hard to get close to break even, particularly when factoring in port days as others have also alluded to. YMMV of course.
A couple of tips and tricks might be:
X occasionally has sales on drink packages prior to sail date. Maybe watch to see if Princess does also?
I think some cruise lines allow you to buy a drink package mid-cruise, pro-rated for the remaining days of the cruise.