To directly answer the questions posed by the OP:
I do think I drink more when buying a package vs. paying by the drink, but I go from having one or two drinks per day to having 3-4 (on port days) or 4-5 (on sea days). The 15-drink limit has no relevance to me, and I have had no regrets buying packages (but I don't always buy them). Regarding the irresponsible consumption, I have only once or twice had a (very mild) negative experience over 60+ cruises (probably 40 or so cruises since drink packages started becoming available).

My general guidelines for choosing when to buy a drinks package:
- Royal Caribbean: Never, as my status gets me 5 free drinks per day - generally beyond what I wish to consume.
- Celebrity: Pretty much never, as my status gets me free drinks during their two-hour "happy hour", again, more than I would enjoy.
(The only exception was when all of their fares included their basic package for a cruise with an ideal itinerary for me.)
- Princess: Sometimes, depending on cruise length and number of ports. The higher the ratio of sea days to port days, the more likely I am to buy their Plus package.
- Norwegian: not sure, although I do not expect to cruise with them again.
- Lines with included drinks but a package to upgrade to pricier alcohol: never.