“Just a question for you: Why is Jesus not sufficient?”
Just a question for you: Have you stopped beating your wife yet?
If you believe the assumption built into your question, you are ignorant of our faith. If you don’t believe that assumption yet still ask such a question, you are like those who questioned Christ just to try and trap him in his words.
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.