I agree that the best method is really to look at the drink package as more of a "What's my break-even" when deciding if to purchase. Of course, you're just projecting what you think you might drink onboard + have to consider how many days at sea vs ported, how many days at Labadee or CocoCay, etc.
I do like the "ease" of the drink package to the point that as long as I think I'll be in the ballpark of my "break-even" point, I'll purchase because I'm on vacation and I don't want to necessarily keep track and think "Oh, well if I drink more than this, then I should have gotten the drink package"--but that's just because I'm a neurotic budgeter, and at the end of the day, I'd rather slightly overpay (because I feel like that's in my hands where I could potentially "make up the difference") than wish I'd gotten the drink package because I was drinking more than what the drink package would have cost me, because I see that drink package $ amount as my "I know I won't pay anymore than this" number.