Once again note that Benjamin's first emphasis is on the brass plates, and only secondarily on the plates of Nephi. One of reasons for this particular emphasis (is spite of the absence of clearly brass plate material outside of the citations of Isaiah) is that Benjamin is highlighting the continuation of the Old World religion among the Nephites with the loss of that religion among the Lamanites. As the direct tie to that Old World, the brass plates both contain the literal and symbolic ties to the God of Abraham, Isaac, and Jacob.