In my experience, cruise lines are very proactive in making sure you have any documentation that MIGHT be needed to reenter the country. Even more so than the airlines for som reasons.
Well, think about it: It's in their best interests to MAKE SURE you're going to be able to get back into the US.
If at the end of your cruise, you can't pass through US Customs, where are you stuck? In the cruise line's terminal. They can't put you back on the ship. They can't let you stay there in the terminal. You're their responsibility because you're in their building.
On the other hand, if you can't get through Customs at the airport, you're in the airport's terminal, which does not belong to the airline. You're on your own. You're not their responsibility.
I always take my passport when I fly - even domestically. I've never been tapped for the additional security screening when I show my passport as my form of ID at the airport. My ex would always get tapped when he used his military ID rather than DL or passport.
To tell an opposite story: My husband was detained for several hours and was taken into the "small room" to be checked recently on a trip to Canada. He says they went through every item in his suitcase, felt the cuffs and seams in his pants, etc. And he was traveling with his passport. When they found nothing, they turned him lose without so much as a word of apology -- he said he wanted to complain, but he knew it was in his own best interest to get out of their as fast as he could. Anyway, the point: A frequent business traveler, he was traveling with his passport.
but the price of most items are not dependant on the number of uses. If you brought a lawn mower or snow blower you couldn't go into sears and say, listen I only plan on using this one time would you lower the price? Or if you buy a dvc time share, you can't go to Disney and say "you know, could you drop the price 5 grand, I only plan on using it for 2 or 3 trips". The price is fixed and the value comes in using it more than once but that does not change the selling point.
Yeah, those things are not priced "by number of uses", but it makes sense before purchasing them to think about how often you're going to use them.
If you're only going to need to mow a lawn only once or twice in the next 10 years, it makes perfect sense to forego buying the mower and just hire someone to mow for you. Similary, if you're only going to take a cruise once or twice in the next 10 years, it makes perfect sense to use the free option.
On the other hand, if you're going to mow your own lawn every week, buying the mower is a good value. Just as if you're going to take other international trips, buying the passport is a good value.
The US is lucky; you have a HUGE volume of space to "play" in without a passport. You've got snow-topped mountains, amazing geological formations, hot beaches, entire cities devoted to theme parks or casinos, desert, lakes and two entire oceans. There's not much space to "play" over here..: that's why almost all Brits own a passport
Which makes perfect sense to me. Also, it doesn't cost you much to travel across an international border. For me, it's quite a distance and expensive.
It doesn't matter how many times you use it, it still comes down to $13.50 per year. Pretty cheap for the best form of idea you can have.
It matters to me. The number of uses determines whether it's a good value for the money. $13.50 per year IF it just sits in the safe is pretty expensive.
On the other hand, if you USE IT REGULARLY, it's a great deal.
To me, that's worth a lot! Yes - I can see it's an initial investment, but $12/year isn't unreasonable by any standards. 
