jayoa, the way you write your post you seem to agree with this state of things. As long as this country doesn't have a national ID card mandatory to every citizen I don't see why one should volunteer to use the passport card as a national ID. I would expect that if citizens are detained to prove their citizenship we'll have some nice lawsuits in the future. However, if we want to turn the US into a police state and demand that everyone carries proof of ID and citizenship with them at all time and it is asked as proof randomly then that would be a different situation (I am not advocating for this).
Well, you can get a passport card and always have it in your wallet. Its the same thing as a passport but in a card format and can be used to travel to some countries yet. It clearly states that you are a US national; http://travel.state.gov/passport/ppt_card/ppt_card_3926.html
This is actually getting to be quite important everyday, which is why they are considering asking everyone pulled over or arrested for their citizenship status regardless if you look or speak like an American or not so as to avoid racial profiling. They may just take you in to find that out or you can make it easy by having a passport card on you. Some states also have "Enhanced Driver License" (aka "Border Pass") can be applied for by for US citizens only and is a joint effort between the state and USCIS. New York is one of four states yet to have it; http://www.nydmv.state.ny.us/edl-main.htm
...here is what it looks like; http://www.bing.com/images/search?q...C2E9BF0E1B630042F5942C933&first=1&FORM=IDFRIR