Nobody is arguing with you about this. What I said was that in some instances USCIS ruled that the Department of State had issued passports by mistake. Logically, the USCIS review has more weight because N-600 is a very detailed form whereas passport application (DS-11) is not.
Not only that, but having a certificate of citizenship is a useful back-up document if a passport needs to be replaced or renewed.
Already under the current State Department rules, if somebody's passport is lost or stolen in the U.S., that person needs to submit DS-11 with evidence of U.S. citizenship - either some original document constituting primary evidence of U.S. citizenship (such as a certificate of citizenship) or secondary evidence,
http://travel.state.gov/passport/get/secondary_evidence/secondary_evidence_4315.html
which for the cases of derived citizenship would mean again submitting evidence of satisfying the requirements of the Child Citizenship Act (which gets harder to do once the applicant is no longer a minor).
In cases of passports lost/stolen while abroad, a U.S. consulate may accept a photocopy of a previously issued U.S. passport as evidence of U.S. citizenship, but for applications within the U.S. they appear to want original documents proving U.S. citizenship, rather than just copies. Even for applications abroad, the practices between different U.S. consulates appear to be non-uniform and they may change in the future to something more restrictive than what they are now.
Also, the State Department has been tightening the rules and internal procedures for issuing passports in the U.S. as well.
The procedures for issuing first-time passports based on the claim to citizenship through the Child Citizenship Act were fairly lax in the past but have been recently tightened.
Applicants whose previous passport was issued before they turned 16 are required to submit DS-11 when renewing a passport, and it is possible that in such cases the State Department already does or will do in the future an extra check to see on what basis the previous passport was issued and perhaps do a re-verification of U.S. citizenship in some cases.
Having a certificate of citizenship is a useful non-expiring back-up document proving U.S. citizenship which could help avoid having to reprove from scratch to the State Department that the applicant has indeed derived citizenship through parents.