You do have to have a social security card if you are born in the US in the last 20-25 years - along with filling out the birth certificate, new parents are required to fill out a Social Security application form.
I would consider it a more valid ID than a passport since it became a required document. You can't enroll in a public school, apply for college, get a job, get a driver's license, file taxes or receive a tax refund, or open a bank account without one (OK maybe there are parts of the country where you can, but I have never encountered any here in the south end of the country). It is used as proof of citizenship already for all of these reasons, whether it's legally required or not.
no subject
I would consider it a more valid ID than a passport since it became a required document. You can't enroll in a public school, apply for college, get a job, get a driver's license, file taxes or receive a tax refund, or open a bank account without one (OK maybe there are parts of the country where you can, but I have never encountered any here in the south end of the country). It is used as proof of citizenship already for all of these reasons, whether it's legally required or not.