As an exercise, allow me to list the differences between a chapter of Isaiah and a chapter of 2 Nephi. Here are the things that Joseph Smith's divine inspiration caused him to change from the King James version of Isaiah chapter 4:
-added a comma after the word "day"
-changed the comma after "saying" to a colon
-changed the colon after "apparel" to a semicolon
-removed the comma after "name"
-changed the comma after "glorious" to a semicolon
-removed the words "shall be"
-changed the word "for" to the word "to"
-added the word "that"
-changed a singular subject to a plural ("they" instead of "he") and appropriate verbs ("are" instead of "is")
-removed a comma after "Zion"
-added "he that" for parallelism
-changed "remaineth" to "remain"
-removed a comma after "Jerusalem"
-added the word "even" for emphasis
-changed the colon after "Jerusalem" to a dash
-removed a comma after "judgment"
-changed "dwelling place" to "dwelling-place"
-removed a comma after "day"
-changed the colon after "night" to a semicolon
-changed "the glory" to "the glory of Zion"
-removed the word "for" before "a covert"
How much of this was important? I'd be willing to bet none of it. Of the twenty-one changes I caught in my side-by-side comparison, twelve of them were inconsequential punctuation changes. The few words that were added or removed were not central to any of the ideas being conveyed in this chapter. Even the pluralized subject in verse 3 doesn't change the meaning of the scripture--it's more stylistic than anything else. The adding of the words "of Zion" in verse 5 doesn't serve much purpose, either, as I would have expected that phrase to be about Zion anyway because of its context.
So in case nobody's clear on the argument that I've been trying to make for several chapters now, there was no point in copying large chunks of Isaiah into the Book of Mormon except to serve as filler, to pad to the word count, or to lend credence to the Book of Mormon by linking it to the Bible. This is not a superior translation. It is the same translation with slightly altered details.
And when I say there was no point, I mean to say that I have concluded that the Isaiah sections of the Book of Mormon are evidence of the book being the words of a man and not the words of God.
They're also easily the most boring parts of the Book of Mormon. (Yes, worse than Omni.)