No, you don't have to register for the scheme and you can take out private insurance or even decide to " pay as you go " The point is that if you did register after listening to the "Scaremonger's" and haven't bothered making payments into the scheme, you could be liable to pay a fine and repay all of the missed monthly premiums.
The question of being stopped at airports came about when one lady asked "will the authorities come after you for this money" the consulates said, Yes, very probably, he said, remember you gave your tax number, this provides links to your bank accounts, residency permit and passport, therefore you could be stopped when entering or leaving the country.
I have to say that I wouldn't be loosing sleep over this, the guy from the consulate seemed less than confident on the subject, I found myself thinking that this guy must have had some sense of the strength of feeling and general confusion about SGK, in fact this was borne out by his admission that similar previous meetings have attracted between 50 to 80 attendees, there must of been 250 people there today, despite this, he obviously hadn't been into the SGK office to agree with the manager what EXACTLY the line is on people who registered fearing a fine if they didn't. telling people that if they went along to SGK, the clerk may take a lenient attitude with you! Understandably, a lot of people who had obviously registered but hadn't taken up the scheme, clearly didn't want to go within a mile of the office.