Having been raised a military brat and having served, I can tell you exactly what a National ID means. When you are required to carry it, then you will be required to present it. On military bases you present it to enter the base, buy gas, buy at the exchange, buy at the commissary, goto the bowling alley, etc.
We don't have a national ID system in place and not everyone is required to have a driver's license, so we don't operate with a national ID mentality. Give everyone an ID and make having it a requirement, and things WILL change.
As for being required to carry something, with a direct biometric system, you'd be hard-pressed not to, and that's important, because it is absolutely unacceptable to require people to carry some ID card around (it would mysteriously vanish whenever some crooked cop or fed felt like arresting you). I fully recognize that sufficiently reliable technology to implement such a system probably doesn't exist yet. But I expect it will before too long, and than I would advocate a system like this. I do not care to have half the population of Mexico and various other 3rd world countries come here and destroy our economy and political system and drive our taxes through the roof, or to have foreign terrorists roaming around at will, working at jobs they aren't entitled to and spending the proceeds on equipment and specialized training to commit various types of terrorist assaults.
Without a means to efficiently and reliably determine citizenship, we might as well abandon the concept altogether and just treat anyone who is physically here as a citizen. That's exactly what the one-world-government crowd wants, and I'm not with them.