Did you hit your head?
A US passport, the book as well as the card, is a CLASS A document. It is the only document that proves not only the identity of a the holder, but also his or her citizenship status.
More to the point, when it comes to the I-9, Uncle Sam does not even accept a Certificate of Citizenship or a Certificate of Naturalization anymore. The only document that proves identity, citizenship, and thus the eligibility to work is the US passport! Why? Because a CoC or CoN has not expiration date and can be faked quite easily, whereas it's pretty darn impossible to fake a US passport.
What does that mean?
It means that the CoC/CoN has only one function: to get the first US passport. Thereafter that passport will serve as proof of citizenship. The CoC or CoN is a purely worst case scenario backup. It may look pretty framed on the wall as well, but is that worth spending $600?
