I've already mentioned that I think the Isaiah-quoting is filler, an attempt to lend the Book of Mormon some (much-needed) credibility, and pretty much unnecessary. But I think I figured out why Joseph Smith decided to use Isaiah as his unnecessary, credible filler: Nobody gets Isaiah.
Let's use this verse from 2 Nephi Chapter 8 as an example (which is extremely similar to Isaiah 51:22):
Thus saith thy Lord, the Lord and thy God pleadeth the cause of his people; behold, I have taken out of thine hand the cup of trembling, the dregs of the cup of my fury; thou shalt no more drink it again.By way of comparison, let's look at Leviticus 12:2:
Speak unto the children of Israel, saying, If a woman have conceived seed, and born a man child: then she shall be unclean seven days; according to the days of the separation for her infirmity she shall be unclean.So, why did Smith decide to quote Isaiah and not something else, like...Leviticus? Because Leviticus actually means something. Isaiah sounds good. It speaks metaphorically of God's broad plans. Other parts of the New Testament are very literal and very specific. And...kind of shocking to the modern-day reader. Quoting Isaiah is a good way of sounding like you're relating important, detailed doctrines without ever really saying anything of consequence.
Which makes it very useful as filler in a new book of scripture.