What does an ID, any ID, do for security? The honest answer is 'not much'. If anything, relying on ID for security purposes actually makes things worse.
Showing ID only affects honest people. If you're dishonest, you can obtain false documents or steal the identity of an honest person.

If a 19 year-old college student can get a fake ID to drink, why couldn't a bad person get one, too? And no matter how sophisticated the security embedded into the ID, wouldn't a well-financed terrorist be able to falsify that, too? The answer to both questions is obviously 'yes'.

Honest people, on the other hand, go to Pro-Life rallies. Honest people attend gun shows. Honest people protest the President of the United States. Honest people fly to political conventions.

Honest people also commute by public bus to work. What if those with the power to put people on a 'no ride' list decided that they didn't like the reason for which you wanted to travel? The honest people wouldn't be going anywhere.
Bad people, besides using fake IDs and stolen identities, can also make the system of checking IDs work in their favor. The Carnival Booth effect, as described by researchers at the Massachusetts Institute of Technology, means that terrorists can probe an ID security system by sending a number of people on innocent trips through the system and noting who is flagged for extra searches and who isn't. They then send only those who the system doesn't flag on terrorist missions.

Still, some Americans think that 'if you have nothing to hide, you have nothing to fear'. Were the Founding Fathers criminals trying to protect themselves when they inserted the 4th and 5th amendments into the Bill of Rights? After all, nobody who hasn't done anything wrong needs to worry about being searched or being forced to testify against himself.
Over the years, Americans have become accustomed to showing ID in any number of circumstances. Few have asked the question, 'Why?'.

The Department of Homeland Security has attempted to institute programs predicated on the use of ID to improve security. The fact of the matter is that demands for ID do nothing for security while making honest Americans less free.