It's better than just trusting the SSN, but I'd still prefer a system that can't be beat with a stick and some chewing gum.
(Yes, I had an ID theft near-miss that involved a locked mail box in a locked stairwell. To me, the more obvious perpetrator seemed to be someone with access to the mail delivery pipeline, but the investigator I spoke certainly did not think terribly highly of the security of a locked mailbox)
With a locked mailbox (which is the default) it becomes a hassle to steal an identity.
If I get a new credit card it's sent to the address I id'd for.