SSN: Identifier or Authenticator?

Spire Security Viewpoint lists some salient points about social security numbers (SSNs), among them this one:

There are over 150,000 people (my estimate) with "defendable" access to your SSN right now. They aren't secret.

— SSNs Re-Re-Re-Revisited, 8 March 2007

And you're ten times more likely, he says, to be victimized with identity fraud by one of these authorized people than by somebody else. And his main point is that the problem with SSNs is not their use as identifiers, rather their use as authenticators. After all, if everybody knew SSNs as readily as names, credit card companies and the like would have to stop using them as authenticators. Then they'd have to use something better for authentication. That would be better risk management.

-jsq