Bottom line is that there may be places with some sort of ID-carry law on the books, but the courts have already ruled against the cops on this one. As a U.S. citizen, you are not required to carry a photo ID at all times (technically, you aren't even required to own a photo ID other than a driver's license) and furthermore, you have the right to not even talk to the police.
I'm not even sure if Missouri makes it law but what happens is up to the officer involved. I was told there is such a law on the books in the state though. I've also talked with others that were issued citations for not carrying a valid ID. Perhaps those get thrown out when they go to court.
If the office does not like something about you, your answer, the way you are acting etc, they can take you in and hold you with no charges at all.
I forget the allowable limit but it used to be something like 21 hours at which time they had to either charge you with something or release you.
Of course they can do that even if you have valid ID on your person.
It is in your best interest around here to carry valid ID for the above reason alone. Usually if you have it and are not acting in a suspicious way, they check you out and you go on your merry way.
I do still hear SSN's broadcast over many of the PD department's radios here. We are still mostly analog and a lot of the small muni's do not have MDT's in their cars so the SSN is how they will do it.
I know some officers for a muni that does not use any form of MDT, they will call in SSN's via cell phone when they have someone stopped that only knows their SSN.
In most cases, as long as the description obtained via the SSN matches the person they have stopped, they let them go unless of course they have warrants.
I've heard situations were there are twins and the officer will take the person in so they can verify identity via fingerprints in most cases just to make sure you are not posing as your twin. For those cases, they only do that if one of the twins does have active warrants.
And yes, this has all became much worse post 9/11. Before 9/11, they would not even stop you if you were staggering down the road late at night as you made your way home from the bar!