As for use in witnessing, if can be effective to get Mormons to start questioning everything else they have been taught about Mormonism.
One can then talk about God being a god of reason, not of unreason. God created man with a brain and a mind with which to think and I believe God expexts us to use it. One can then go into the factual basis and historical evidence for Jesus Christ and the Bible. I will then sometimes go into the fact that there are hundreds, if not thousands of historical records and archaeological cites confirming the Bible, but not a single one in support of the Book of Mormon. From ther, a simple sharing of the Gospel works.
Invite them to one of your community services, and SHOW THEM the LOVE OF GOD at work in your church (if it is there :-).
That is how I was converted. I was led away from Mormonism by having the truth about the LDS Organization and beliefs revealed to me and I was led to Christ by the love and compassion of people who showed a genuine interest in and concern for my eternal destiny. I was not instantly led to Christ merely because someone proved my LDS beliefs were wrong. In fact the revelation of the errors and historical truths about Mormonism merely made me an atheist. It was the love of God, which was manifest in the people who were praying for me and proselytizing me, which brought me around to Christ.
And, BTW it was hearing and responding to the dreaded "Revelation 3:20" that brought me to my knees, a verse Calvinists insist does not apply to those who are not already saved. But then how do they explain the uncountable numbers of people (myself included) who are now Christians because they heard that verse and responded?