Joseph Smith said that the Scriptures (the Bible, not their BoM, PoGP, or D&C) are true "insofar as it is correctly translated"
So if we present an essential doctrine we believe based on Bible passages, and this doctrine is not held by the Mormon prophets, then they simply right it off as incorrectly translated. It's an easy way to dance around the doctrines so important to our salvation.
I spent several years studying Mormon as well as Watchtower teachings, visiting their institutions and libraries, eventually collecting my own library of both cult's publications (some go back to the early 1900s). These are very handy to have for anyone wanting to dialogue with them.
I have a friend who worked at the JW headquarters (the actual Watchtower Bible and Tract Society), he came to the truth through a private study of Romans (which is forbidden by the WT), today he has a ministry to help ppl out of cults*. Anyways, he said the key to opening a cult member's mind to eventually hear what the Bible says, is to shake their foundation - show them that their teachers or prophets have lied, had discrepancies and contradictions. Once they see the fallacies from their own publications, only then can they be receptive to what the Bible has to say.
I have personally done this over the years. I've photocopied pages from their publications and showed them, when they say that the photocopies are phony, I either show them the original I have or tell them to take the copy and verify it's authenticity from their own archives and get back with me. I've had some who were receptive, many who flat out turned me down. (I got a lot of great stories !)
*http://freeminds.org/ (Randy's goal is to free ppl from cults, not necessarily win them all to true Christianity... so his sight is for the open minded). He's a great guy, a people lover, I've bought and been given a bunch of books from him.