You'd be surprised: at recent interview HR manager agreed with me that at THAT moment she did not need my SS#. That happened, though, only when I got up and was ready to leave. She asked me to stay and agreed that the number is needed only for taxation purposes - employer is a tax agent for the employee. When the interview came to unsuccessful end, - they couldn't pay what I wanted, - we parted company on that there was no reason for my SS# to sit in their, on any other potential employer, BD forever. That, BTW, goes for all other personal info: DL#, DOB, etc.
That is THE SCRUPULOUS company or, personally, that HR manager. Others just use their dominant position to poke their noses where they should not. Anyway they are not gonna do a background check on ALL applicants. Job offer is conditional on all bells and whistles they want. Before that they should be interested only in my personal/professional suitability for the job. Going beyond that is UNSCRUPULOUS, IMHO.