The referent for "they are of worth" is the previous verse's reference to the words of Isaiah. It is the words of Isaiah that are of worth to the children of men. It would appear that even in Nephi's day there were those who found Isaiah laborious reading, and even some who questioned the value of Isaiah. Nephi notes that he is speaking to his own people, and his explanation is for them. Modern readers do well to take heed to his counsel, for we frequently have some of the same opinions.
It is not surprising that there would be those among Nephi's people who would see no value in Isaiah's words. First, his people do not understand the prophetic style of Isaiah, as Nephi has made clear in this introduction. Secondly, the majority of his people are now probably more a New World people, with only the stories of the leaders and perhaps parents to tie them to the Old World. If, as suggested, there are many in the community who have no Old World connections at all, and are truly New World people, it is quite reasonable that they would question the applicability of Isaiah's pronouncements upon Israel to their own lives and destinies.
Nephi's view of Isaiah not only has ties to the Old World but his view is one in which he insists on a direct application of the prophecies to his own people. In the prophecies of restoration, Nephi sees his people foretold and it is this inclusion of his people in the promises to Israel that he intends to expound.