we see the mug double benefit, yes, we likely drink $15 worth of coffee/tea/hot choc and soda over the course of our vacations (which also could lend to this discussion... are you a four day per visit guest or a ten day at a time guest??) per person, PLUS we consider it a souvie... and yes, I have more than five "exact same mugs" in my cabinets and yet, when most are in the d/w, the kids fight over the remaining ones... we like them both on vacation and at home... so for us it's not some "extra widget that we won't use" but rather a valuable component of the DDP.
The DDP is definitely not "one-size-fits-all" and may not work for every guest... your mileage may vary... however, that being said, there is no blanket statement that "DDP is a waste, you can never come out ahead" because you most definitely can!
Also, we do not stress about getting the most bang for our buck on the DDP... if my teen kids want kids' chicken fingers, so be it... once I'm on vacation, I don't care if I got my nickel's worth or not... and I like having it prepaid, rather than putting it on a credit card when I'm there... nothing takes the wind out of one's sails faster than a big bill at the end of a vacation!!! Even on our
DCL we prepaid everything including gratuities, shore excursions and put money on our onboard account prior to sailing so as not to leave with a bill as our souvie!