Then scrap the book Joseph Smith wrote and stick with the one written by Jesus’ friends and those who prophecied His coming.
By keeping Joseph’s book(s), you are saying that Jesus is insufficient, and that you need “another gospel,” something *more* than what Jesus said and accomplished.
Would you toss out the Gospels of Mark, Luke, and John just because you have Matthew’s account? Our motive for using the Book of Mormon is rooted in our belief that it is the word of God and a testament of Christ. We love the word of God, all of it, and we expect God to give us more of his word as time goes on.
If you are honest at heart, you will recognize that our motive for using the Book of Mormon is a love for the word of God, even if you think we are mistaken about where the word of God can be found. Attributing other motives to our using it is just spin and manipulation.