The New Testament is the reason why we need Prophets and Apostles. Look in Romans, Ephesians, Matthew, Mark, Luke, and John.  
 
Christ built his church on the foundation of prophets and apostles with Christ being the chief cornerstone. We need prophets for the same reason that the ancient Church needed Peter, James, and John to guide the Kingdom of God on the Earth through the turbulent waters of modern society. To prevent the members from being tossed by every wind of doctrine.  
 
Yes there will be false prophets we can see them on ever tele-evanicgal program on the TV, send me money and God will bless you, pay this much and God will forgive your sins. The wolves are among the sheep and are deceiving so many with clever teachings of men mingled with scripture.  
 
The Book of Mormon though ends the twisted false teachings of men though. With only the Bible you can have an infinite amount of interpretations of the scriptures, just like you can have an infinite amount of lines that can be drawn through a single point.  
 
The Book of Mormon though give us a second source of confirmation and guidance that strengthens and enriches our understanding of the Gospel. Take that mess of a first dot with an infinite amount of lines or interpretations and add a second reference point (Book of Mormon) now there is only one line that can be drawn between the points and there is no more confusion or false teachings.  
 
The Book of Mormon alone can be interpreted millions of ways, the Bible alone can be interpreted millions of ways but the two of them standing together as duel witnesses of Jesus Christ's device mission and calling as our Lord and Savior leaves no room for false doctrine.  
 
This is why Heavenly Father restored this record to the Earth when he was ready to do so, it took the various man made doctrine of the Churches that had arisen in the Great Apostasy and laid them to rest once and for all with a second witness from the other sheep that Christ was called to visit.  
 
As for the false accusations that the Book of Mormon contradicts the Bible, please provide evidence of this weak and pointless claim. There is no contradictions, to find them is to also have to scrutinized the New Testament to the point that it too is invalid for contradicting the Old Testament.  
 
In the 181 years since the Prophet Joseph Smith translated the Book of Mormon tens of thousands of critics have tried to tear down the Book of Mormon in order to refute and invalidate the Church of Jesus Christ of Latter-day Saints which the Lord built upon the rock of this Restored Gospel. In those many years none have succeeded or come close to diminishing it's great influence and power to bring mankind to the Savior Jesus Christ.