Check out this story about ICE illegally detaining and deporting U.S. citizens:
http://www.startribune.com/nation/14456137.html
It is stuff like that which makes me think that carrying a proof of U.S. citizenship on you may be a sensible precaution in this day an age...
Actually, one of the main reasons I got a passport card was to have it with me when I travel abroad, so I have an extra original document proving U.S. citizenship., in addition to a passport book. If a passport book is lost or stolen while one is travelling abroad, technically the instructions on the websites of all U.S. consulates do require presenting an original (rather than a photocopy) document proving a U.S. citizenship in order to obtain a new passport. In practice they do actually accept copies, but, based on the recent experience of a friend of mine whose passport was stolen in France, the procedure is a bit more cumbersome if no original document proving U.S. citizenship is available.