With the current ID system, security is only as strong as the weakest link... you forge whichever state has the least security measures, and use that. I really don't see the difference, aside from the potential money savings of 1 entity designing and implementing security measures as opposed to 50.
Nowhere was it ever said that you would be required to carry the card... just as currently, you don't need to have a driver's license/state ID... it's just that if you get into some trouble with the law, you've got a good chance of going through a lot of hassle that could have been prevented had you been able to identify yourself.