I first heard of the U.S. Passport Card last year, and I assumed that it was an ID document that was specifically offered for the convenience of populations living along the Mexican and Canadian borders.  
That’s still the stated purpose on the State Department site, but there’s at least one account of it being pushed as a low-cost alternative to a regular passport. Â This article over at Matador Pulse does a good job of running down the reasons that it’s a bad alternative to a regular passport (tho’ the concern about the card’s RFID chip applies equally well to regular passports).
