The term “fullness of the scriptures” does not just appear in the Teachings and Commandments. It is also used by Christ in the New Covenants in the Book of Luke.
A lawyer confronted Christ about the reproach Christ addressed to the Pharisees, saying, “Master, thus saying, you reproach us also.” Thereupon Christ expounded on how abused the law had become under their stewardship. In His condemnation Christ stated: “Woe unto you lawyers, for you have taken away the key of knowledge, the fullness of the scriptures. You enter not in yourselves into the kingdom, and those who were entering in, you hinder.” NC Luke 8:17
This idea is echoed in the revelation to Nephi: “because of the many plain and precious things which have been taken out of the book — which were plain unto the understanding of the children of men, according to the plainness which is in the Lamb of God — that because of these things which are taken away out of the gospel of the Lamb, an exceeding great many do stumble.” NC 1 Ne. 3:22
The Joseph Smith Translation of the Bible was not optional. Without it no one can enter into the kingdom. The fullness of the scriptures (JST Bible) is a mandatory restoration of what was in the heart of the Lord. T&C 18:6
The new scriptures are essential for believers to be able to recover truth and proceed forward. Without them we have no better chance of seeing Zion than the residue of Christianity.