The Passport Card is particularly useful if you live near the border with either Canada or Mexico. I live on the Mexican border and it is quite common here to go on short same-day errands into Mexico and back. I have both the Passport book and card (and btw, “La chica gomela”, I am also an expat in Europe most months out of the year, and it was precisely through a US Embassy in Europe how I got my card), and let me tell you that if you live on the border it is far more convenient to carry the card with you rather than the book. The passport card is the same size of a credit card or driver license.
Another benefit of the Passport Card, is that it is a cheaper alternative to the book. It costs $45 the first time, valid for 10 years, and to renew it thereafter costs only $20 (my Texas Drivers License is more expensive at $26 and on top of that it is valid for only 6 years.). Otherwise, paying the steep $100 fee for the book would be too much in a region with a lot of people living below the poverty line (and who would thus be less likely to travel overseas). Imagine a family of 4 or 5. Multiply that by $100, and it becomes too expensive… especially if all you need it for is for short same-day errands across the border.
Sorry I made this rather long, but I felt I had to comment on the Passport Card and its usefulness… I assume some people here (esp. “La chica gomela”) do not live anywhere near a US border and therefore might overlook the usefulness of the Passport Card.
Bottom line, the Passport Card was designed with the needs of border residents in mind.