D&C 68:4 And whatsoever they shall speak when moved upon by the Holy Ghost shall be scripture, shall be the will of the Lord, shall be the mind of the Lord, shall be the word of the Lord, shall be the voice of the Lord, and the power of God unto salvation.
Such things are the mind of the Lord, the will of the Lord, and the power of God unto salvation. But the message in that scripture is that such speaking is what we must heed and do to gain salvation. It is not that whenever a man speaks by the power of the Holy Ghost the words that he says are perfect even as god is perfect.
Joseph Smith was bold to ascertain the truth of what he taught by the power of the Holy Ghost. He stated "I ... know more than all the world put together. The Holy Ghost does, anyhow, and He is within me, and comprehends more than all the world: and I will associate myself with Him." In fact, he even said that he would teach the Elders and the Elders would teach the saints. "... I must teach the Elders, and they should teach you. God made Aaron to be the mouthpiece for the children of Israel, and He will make me be god to you in His stead, and the Elders to be mouth for me; and if you don't like it, you must lump it."
Nevertheless, Joseph Smith seemed to clearly distinguish between his teachings and God's revelations. This implicit in much of his counsel:
"Search the scriptures--search the revelations which we publish, and ask your Heavenly Father, in the name of His Son Jesus Christ, to manifest the truth unto you, and if you do it with an eye single to his glory nothing doubting, He will answer you by the power of His Holy Spirit. You will then know for yourselves and not for another. You will not then be dependent on man for the knowledge of God; nor will there be any room for speculation. No; for when men receive their instruction from Him that made them, they know how He will save them. Then again we say: Search the Scriptures, search the Prophets and learn what portion of them belongs to you and the people of the nineteenth century." (TPJS page 11)
D&C 67:9 For ye know that there is no unrighteousness in them, and that which is righteous cometh down from above, from the Father of lights.
While all scripture is true, it is worth knowing when some part or other reflects truth in a particularly potent way. We should expect that to be the case. Christ distinguished some statements by prefixing them with "verity" and others with "verily, verily" and others not at all. Verily means truly. Some statement he made struck at the truth in a way that was more fundamental and eternal that others. Some principles were true, like the word of wisdom is true - God requires it of us - and others struck at the heart of eternal truth.
We learn line upon line, and precept upon precept, and sometimes a statement reflects the truths of eternity peculiarly potently.
Now the point I am getting at is that when King Benjamin taught his sermon he was speaking under the influence of the Holy Ghost, but one chapter stands out above the rest, which was given to him verbatim by an angel of God. And when we read the teachings of the prophets in the scriptures the vast bulk of it is actually a man speaking or writing under the influence of the Holy Ghost. But there is a difference between a man simply writing or teaching by the Holy Ghost and what God himself will say. And the revelations of Joseph Smith are of this latter type. They are dictation from God. There is no imperfection in them.
They are perfect as God is perfect. They are dictation from him.
Perhaps I can explain it this way. Assuming you and I press forward diligently as we ought and we are true and faithful in the days of our probation. Then one day we will know as much as Alma did when he spoke to the people, or as King Benjamin did when he gave his sermon. When that day comes when we read his words we will agree with them as right and proper but will also think to ourselves "That seems like the sort of thing I would have written". And one day we will know as much as Joseph Smith did when he taught the Saints and when we read his writings we will think to ourselves that his teachings were right and good (and limited by the Saints willingness to learn) but it will become the sort of thing we could have taught if we had been there at the time. But even when we get to that point, well after our mortality has ended, when we read the revelations from the Lord we will find that there is no imperfection in them and that they still stand far ahead of what we then fully comprehend and know. We will find they contain goodness still that is far beyond the goodness we will have then reached.