The effect of reading the scriptures of the brass plates had a profound effect on Lehi, sufficient to induce prophetic vision. While the scriptures may always have a similar effect on all of us - to inspire us to greater receptivity to the things of the spirit - the brass plates had a particular strength for Lehi due to their relationship to his ancestry, and the Spirit-guided means in which they were delivered to him.
It is interesting that when Nephi records that Lehi began to prophecy that Nephi limits his record of that prophecy to that which concerned the plates themselves. Surely what Lehi had to say about his seed (v. 19) would be more interesting to Nephi than a general prophecy on the effect of the brass plates. Nevertheless, all we have recorded is that the plates would "go forth unto all nations, kindreds, tongues, and people who were of his seed." This prophecy appears directed at the Nephites more than the modern recipients of the Book of Mormon, though that new book should surely continue the prophecy of delivering the words (or at least some of those not present in the biblical canon) to all nations..
The prophecy that the plates would no longer be dimmed by time has two possible references. One is that the words of the plates would be preserved and be present in the minds of his descendants. This would contrast with Lehi's general knowledge about the plates before receiving them, but his lack of specific knowledge (witnessed by his wonder at reading them).
The second is a near magical contention that the plates themselves would no longer be "dimmed by time", that is, that the effects of time would not diminish their sheen. To be read in this literal sense, we would have to accept some tarnishing of the plates already "neither should they be dimmed any more by time" (v. 19). Perhaps they had required cleaning from during the years of storage, and from then on were miraculously kept free of age. As I know of no further reference to their miraculous state of preservation, I prefer the former reading.