'The question is not "why bother with another text""'
It's my question. Maybe the OP will not find it awkward, and can answer it in a fashion that will be intellectually acceptable.
It's not a matter of whether the question is awkward.
Obviously you missed the point of the rest of my post. If Text A is held to be sacred, and Text B expands and expounds upon Text A without contradicting it, why bother with Text B?
That same question seems to have different answers for you depending on whether Text B is the New Testament, a given book of the Bible, or the Book of Mormon. All are additional texts that do not contradict other works; the Gospels, especially, seem to be redundant.
An additional text can completely change the meaning of a previous text without contradicting anything from the other text. The Book of Mormon seems to do this, just as the books of the New Testament did so with the Torah. Just as Mark did with Luke.
Again, the
valid question is not "why bother;" we
know why one would include additional non-contradictory texts: additional texts can add additional information and change the way the older text is interpreted.
The valid question is "is this new text authentic? Is it accurate?"
If the text can be shown to be a forgery (purporting to be old when it can be shown to have been written recently, for example), or if it is inaccurate (claiming that an ancient tribe of Hebrews, for example, migrated to North America, a claim that has ben thoroughly falsified),
those are valid reasons to exclude a text.
Simply saying that any additional text that doesn't contradict other texts is extraneous required special pleading: the Bible you accept is itself a collection of books that presumably do not contradict one another, but instead add to what was written in other texts. If you exclude the Book of Mormon as extraneous on thsoe grounds, it's inconsistent of you to include more than a single book from the Bible.
A book can only be considered extraneous if it
adds nothing new. The Book of Mormon most definitely adds new information, and so cannot be regarded as extraneous.