In my experience, it is always quicker and more efficient to have additional passport pages inserted at a US Embassy or Consulate than at a passport office in the US. No appointment is necessary and, with a bit of luck one is in and out in less than an hour.

On the original subject of this thread, I too find it strange to have my US passport stamped upon re-entry into the US. Some border control agents stamp it and others don't. The stamp is a generic stamp that is used both for US citizens and foreigners coming to the US. The stamp says "Admitted".
In contrast, the EU prohibits stamping EU passports upon entry into the EU because it deems a passport stamp an imposition of a control that the person is not subject to.