The truth is that if passports as a national ID were found to be compromised for the purpose of voter fraud or medical or social security fraud, it would no longer be trusted and accepted by other countries.
That's why you'd never see passports used as national ID's.
Politicians want an internal ID that they can compromise for their purposes without concern for international treaties.
-PJ
Hmmmmmmmmmm, that sounds like a good reason to require one for anything important, like renting a movie or voting...