For some time, I have been jealous of my European friends, who travel within the European Union with just their national identity cards instead of the bulky, traditional, as they call it, “international passport.” But then I realized we have the same thing, the US Passport Card, which is only billed as an alternative for traveling to Canada, Mexico, and the Caribbean, but turns out to be much more useful.
Between my arrival from the US and my return, I'm required to have my passport with me at all times, and it actually does get used often, most notably it's needed when checking into a hotel or renting a car. I have found that unlike a state driver's license, the US Passport Card is universally accepted by clerks and even the police. This is where the passport card really shines, especially if you have a “home base” (or a hotel safe.) Just leave your passport book there, and slip the card in your wallet. In fact, even when my passport book is with me, (normally in my laptop case) I'll whip out my passport card from my wallet since it is handier.
As a man, I only carry a thin wallet and a phone around with me; no fanny pack or European man-purse, thank you very much! Therefore, prior to getting my passport card, this meant my passport book often found itself in my back pocket getting frayed, damp, and bent—not a good thing for a machine-readable document. Also, it was kind of risky dangling this, my ticket home, out of my back pocket. Now, with my passport card in my wallet, the book stays in a safer place.
If you ever do lose your passport book during a trip, which thankfully I haven't, I suspect that having a passport card is going to make it easier to get to the nearest US Embassy or Consulate, and having a replacement passport book made quickly. But, in any case, by carrying the passport card day-to-day (not crossing international borders,) the chance of having your passport book lost or stolen of course decreases.
As an aside, notice the suit and tie in my mugshot; this is my new thing for passports, and I think I'll go to the DMV dressed up next time too. My thinking is this: whatever the situation, whatever authority looks at this ID will—in at least in a small, unconscious way—hold me in higher regard as a respectable professional regardless of what I'm wearing at the time. Incidentally, you don't have to rely on the local drugstore employee to take your next passport photo; there are several resources on the web that will let you take any recent picture (just make sure you are facing and looking into the lens in front of a light-colored background) and automatically scale/crop it so that you get 4 passport pics on a standard 4x6 print that might only cost you 9 cent.
After you have the photos, just fill out and print this application found at the State Department website sign, enclose $30 check, and your passport book in a US Postal Service express mail envelope and mail it off for $19.95. Within two week you should received you passport book back and, separately, your shiny new US Passport Card!
To summarize, the US Passport Card is handy complement to your passport book; it will not get you across an international border (expect Canada, Mexico, & the Caribbean) but it's incredibly handy during your stay abroad, and definitely worth the effort if you travel abroad regularly.