Mormons believe in the Bible, which teaches that God is infallible. They believe the Bible is God's Word. They also believe that the Bible is not sufficient, that God made a mistake and had Joseph Smith write the BoM. It's a little contradictory.
Wow...so many leaps.
God may be infallibe, but men aren't. And as so many have mentioned, the many, many versions of the Bible are compilations made by men of various writings which were written many years, decades, centuries AFTER the events. And in fact, I think most scholars would agree some parts of the Bible are legends which illustrate principles, but are not necessarily historical. And which books are to be included in the "Bible" and which are not? Personally, I have a problem with the Song of Solomon, which is nothing more than soft-porn. Different versions of the Bible have different books, as well as differing translations, which often lead to different interpretations.
If there is a God, he did not make a mistake, but many men may have in choosing which books to include, in translation, etc. And if you are to believe Joseph Smith, God did not have him "write" the BofM. It was something he translated. It may be far-fetched, but I don't find it contradictory. Should the word of God have stopped after the Pentateuch? Should the Ten Commandments and the Law of Moses have been the be all and end all of God's word? Or maybe there was a need to supplant and supplement with the teachings of later prophets, including Christ.
To me...if I accept there is a God...it seems strange that "revelation" would have stopped with the death of Christ and his apostles. In 2000 years, God has absolutely nothing else to say to us? If we are left to navigate the many new issues which have arisen since the passing of Christ by simply relying on the various councils of religious leaders - sans revelation or inspiration from God, then I may as well not believe in God, at least not on a personal level. If that truly is the case, then a Higher Power might be just some mystical energy, responsible for the life force of the Universe, but not for any of us individually. That's just too depressing for me to believe. Call me weak, but the ONLY thing that gives true meaning to my life is my family. And if I were to lose that belief, I'd probably lose the reason to keep living.