It is not unusual for Latter-day Saint commenters to suggest that Nephi is offering a commentary on Isaiah. That is not what he is doing, and he declares as much. What he has said is that Isaiah prophesied, and that Isaiah is difficult for one to understand without having the spirit of prophecy. Nephi declares that his spirit of prophecy delights in plainness, and here declares that he will proceed with his own prophecy, and do so in that plainness. This is not a commentary on Isaiah, but will be similar to the way Jacob used Isaiah. Nephi will be giving his own prophecy for his own people, but touching Isaiah as a foundation upon which Nephi’s prophecy is built.
In verse 8, the things which are of worth unto the children of men is the ultimate part of Isaiah’s prophecies. Nephi knew that the Assyrian invasion had happened, and that Babylon had invaded. He knew by his father’s revealed declaration that Jerusalem had been destroyed. None of these elements were still prophecy from Isaiah. What continued to be prophecy was the theme of the return and the final victory of the Messiah. Those are the things of worth to Nephi’s people, either to those present with him or to those reading his words in the future.