The really fast answer is, we don’t. However, this is a big question for a lot of people who have read these verses at the end of the Bible.
18 For I testify unto every man that heareth the words of the prophecy of this book, If any man shall add unto these things, God shall add unto him the plagues that are written in this book:
19 And if any man shall take away from the words of the book of this prophecy, God shall take away his part out of the book of life, and out of the holy city, and from the things which are written in this book.
The part people don't understand and misinterpret about these verses is that John, the author of the book of Revelation, was alone on the isle of Patmos when he wrote and completed this visionary work. The Bible as we know it today with 66 books wasn't put together for hundreds of years. Those verses aren't referring to the Bible, that it cannot have words added or taken from it, but that nothing should be added to John's revelation or taken from it. It was meant by God to survive intact without change so that those in the last days would have the benefit of studying and knowing what had been prophesied.
Further evidence of this understanding comes from the writings of Moses in Deuteronomy 4:2 which says:
2 Ye shall not add unto the word which I command you, neither shall ye diminish ought from it, that ye may keep the commandments of the LORD your God which I command you.
If the interpretation of Revelation was as some in the world declare it, they would seal the heavens and prevent any further words from God from being given to us. However, if that interpretation were correct, then Moses' words would have shut off all communication from the heavens as well. Where would Isaiah have fit in or Ezekiel and all the other prophets? Such is not the case. God calls prophets and speaks to them, using them as his mouthpiece on earth to guide his wayward children.
From the Book of Mormon, one ancient prophet was commanded to write these words from the Lord.:
2 Nephi 29:3-11
3 And because my words shall hiss forth-many of the Gentiles shall say: A Bible! A Bible! We have got a Bible, and there cannot be any more Bible.
4 But thus saith the Lord God: O fools, they shall have a Bible; and it shall proceed forth from the Jews, mine ancient covenant people. And what thank they the Jews for the Bible which they receive from them? Yea, what do the Gentiles mean? Do they remember the travails, and the labors, and the pains of the Jews, and their diligence unto me, in bringing forth salvation unto the Gentiles?
5 O ye Gentiles, have ye remembered the Jews, mine ancient covenant people? Nay; but ye have cursed them, and have hated them, and have not sought to recover them. But behold, I will return all these things upon your own heads; for I the Lord have not forgotten my people.
6 Thou fool, that shall say: A Bible, we have got a Bible, and we need no more Bible. Have ye obtained a Bible save it were by the Jews?
7 Know ye not that there are more nations than one? Know ye not that I, the Lord your God, have created all men, and that I remember those who are upon the isles of the sea; and that I rule in the heavens above and in the earth beneath; and I bring forth my word unto the children of men, yea, even upon all the nations of the earth?
8 Wherefore murmur ye, because that ye shall receive more of my word? Know ye not that the testimony of two nations is a witness unto you that I am God, that I remember one nation like unto another? Wherefore, I speak the same words unto one nation like unto another. And when the two nations shall run together the testimony of the two nations shall run together also.
9 And I do this that I may prove unto many that I am the same yesterday, today, and forever; and that I speak forth my words according to mine own pleasure. And because that I have spoken one word ye need not suppose that I cannot speak another; for my work is not yet finished; neither shall it be until the end of man, neither from that time henceforth and forever.
10 Wherefore, because that ye have a Bible ye need not suppose that it contains all my words; neither need ye suppose that I have not caused more to be written.
11 For I command all men, both in the east and in the west, and in the north, and in the south, and in the islands of the sea, that they shall write the words which I speak unto them; for out of the books which shall be written I will judge the world, every man according to their works, according to that which is written.
As further evidence of this, consider that at the time the 10 tribes of Israel were taken captive by the Assyrians and then later escaped and were lost to our knowledge, it was during a time when there were many prophets among the Israelites. Who would dare say the words of the prophets that were with the 10 tribes were invalid just because they are not in the Bible we hold today? There are even several writings of prophets mentioned by name in the Bible which we do not have today (ex. 1 Chron 29:29).
Lastly, in the Book of Revelation it speaks of 2 witnesses that God will give power to, to prophesy at the battle of Armageddon. Are these not prophets who will utter the word of God in great power? Will they not declare the word of the Lord just as surely as Moses or Isaiah or Peter or Paul? When their writings are written will not people regard it as modern day revelation worthy of canonization? I think so.
Mormons don’t exclusively use the Book of Mormon. We use the Bible for its testament of God’s dealings in the ancient world and the Book of Mormon for its testament of God’s dealings in the ancient Americas, the Doctrine and Covenants as modern day revelation, and the Pearl of Great Price which was revealed to the prophet Joseph Smith. We regard all scripture as valuable for our profit and learning.
In the LDS church, we believe in modern revelation. We believe that God wants to speak with us today and has called a prophet again on the earth to show that he loves all of his children, the same today as he did anciently. If there were times in the past when God's children needed inspiration from God, we certainly can't argue we don't need that today. From Joseph Smith's first vision of God the Father and His Son Jesus Christ, to Thomas S. Monson today, God has called prophets to guide and direct his affairs on earth.