I have to admit: as a child/teenager/young adult, I found the value placed on having a testimony of Joseph Smith...well, somewhat troubling. Shouldn't there be a lot higher emphasis on having a real, living testimony of the Atonement or of God's love for us? After all, Smith himself taught of the Atonement: "all other things which pertain to our religion are only appendages to it" (Teachings of the Prophet Joseph Smith, 121).
I tabled this concern for a long time, and tried to focus my studies on doctrines I considered more important (principally the plan of salvation, the Atoning sacrifice of Jesus, and God's love for me).
Recently, I was sitting in stake conference and something clicked into place.
"a pillar of light above the brightness of the sun at noon day came down and rested upon me and I was filled with the spirit of God and the Lord opened the heavens upon me and I saw the Lord and he spake unto me, saying 'Joseph my son thy sins are forgiven thee. go thy way walk in my statutes and keep my commandments behold I am the Lord of glory I was crucified for the world that all those who believe on my name may have Eternal life.'" (Joseph Smith's first vision, 1832 account)
"And now, after the many testimonies which have been given of him, this is the testimony, last of all, which we give of him: That he lives! For we saw him, even on the right hand of God; and we heard the voice bearing record that he is the Only Begotten of the Father— That by him, and through him, and of him, the worlds are and were created." (Doctrine & Covenants 76:22-24)
The testimony of Joseph Smith is not a testimony of an extraordinary man, an man worthy of veneration (beyond the admiration we ought to give to those who humbly, obediently give all they have to the God they love). It is a testimony of the Father and Son that he bears. When I read these words, I know that God lives. I know that I can be forgiven of my sins. I know that I can find peace and strength beyond my own through the grace of the Atonement.
In the end, it's not a testimony of the man himself that we should obtain, but of his testimony of God/Jesus. As we come to know God, we will recognize the truthfulness of the witness that Joseph Smith bears. Because the words of Joseph Smith provide such a true testimony of God and of Jesus, we can know that he was called of God.
