First Nephi 5:19

Wherefore, he said that these plates of brass should never perish; neither should they be dimmed any more by time. And he prophesied many things concerning his seed.

The brass plates are the source document from which Nephi, Jacob and others quoted old world scripture in the Book of Mormon. They were all the scripture they had, and they cherished them. Lehi prophesied that someday we will have these plates. It would be wonderful to have these additional prophecies and revelations, but only if we have put to good use the word of God we already have in the Standard Works.