Just because you shouldn't use it as an identifier doesn't mean you don't need that information in the general case. SSN as identifiers are bad in the case of:

Just because you shouldn't use it as an identifier doesn't mean you don't need that information in the general case. SSN as identifiers are bad in the case of: non-citizens, babies, and fraud (may have multiple people with the same ID). However, if you are dealing with government services (especially anything that touches on taxes or benefits) you'll probably need to provide a valid SSN. And then also be prepared to deal with oddball cases too.