I, for one, do not carry my passport on me everyday.
I only have it on me when I am in between accommodations or day-tripping. It is a matter of choice and opinion, regarding this practice... and I am not alone in doing so.
Most of us know that this practice is not in accordance to the strict interpretation of the law in some places. It is good to know that this card would NOT be a substitute for a passport, again in the strict interpretation of the law. However, for all of the other RS Forum readers who do as I do, and hold valid U.S. passports, this is an interesting card to hold, IMO. I have used my Canadian Citizenship card in the exact same fashion.
For years now, my Citizenship Card has been 'casually' regarded as an "Official National" piece of identification by everyone wearing a uniform in a country that does have such a thing. This has been my experience when I have had to produce an "official" piece of ID (National logo + photo + full name + ID number = a pretty good substitute). Yet, it has really no value at all. None. Most Canadians don't even know it exists, LOL!
Don't be so harsh on the OP.
If I were a U.S. citizen, I'd be glad to find out about this and I would get one. I'd find it much more useful, and would feel better having it on me, in addition to the photocopy of my passport. Especially given that - gasp - I make the choice to never carry my passport on me, on most days when I am in Europe.
Again, this card is no substitute for the times when a Passport is required. But it would be good for 'casual' presentation if needed, with the possibility of producing the actual passport fairly quickly, if needed. My opinion and a seasoned traveller's choice.