And the reason for the SSN was so that IRS could track every return you prepared, and do a "practitioner project" if it found mistakes on too many of them.  

Anyone can find a list of a hundred preparers within five miles of my ZIP Code.  Then they can rent a storefront and use my identity to prepare returns from January through April 15.  They can't find my PTIN, but they can make one up and they'll be gone before IRS notices.  Of course, if they want to do e-filing the scam will have to get more complicated.  Better just to tell folks it was e-filed, and mail it in.  Move on to a different ZIP Code next year.  

Protect your PTIN?  You might as well try to protect your license plate.