I have both the traditional passport and the card... but understand that the card is only a convienient ID. It isn't reccomended purely for travel because numerous countries are unfamiliar with it. They need the book to stamp.
If I get carded or asked for ID locally tho I'll show my passport card, especially to pigs. I'm clearly a US citizen, so it lets them know I have both the means and oppritunity to travel which in turn lets them know that I may be in a tax brackett that can cause trouble for them should it go that route.
They like to harass ppl without means, but hesitate to bother ppl that can defend themselves. Most ppl in the hood don't have passports, so when the filthy pigs see one whilst they are making their harassment rounds it is an anomoly. They usually move on.