The Book of Mormon records some of God's word on the issue of additional scripture. I recently re-read this passage of scripture, and was impressed by several points made.
Additional scripture helps in the gathering of scattered Israel:
God begins in 2 Nephi 29 by giving some of the reasons for addtional scripture. He reminds us that part of the reason for bringing forth scripture is to fulfill covenants He made, that scattered Israel would be gathered. In addition, the writers of the Book of Mormon had prayed that God would one day gather again their own posterity if they dwindled in unbelief. The Book of Mormon was written for our day, and specifically for the descendants of ancient American prophets, to help them come unto Christ. It is interesting to watch the conversion of people in Central and South America as they read the record of their ancestors. They accept the gospel whole heartedly, and turn their lives to Christ. The Book of Mormon has been a great tool in helping to gather scattered Israel.
Is the Bible enough?
Continuing on in 2 Nephi 29, God questions those who say there is no need for more scripture than a Bible. He makes the point that the Bible came from the Jews, but the world at large, Christians included, largely ignore the sacrifice made by the Jews to preserve the scriptures for us: