“These stones, the Urim and Thummim which were given to the Brother of Jared, were preserved for this very purpose of translating the record, both of the Jaredites and the Nephites.
Then again the Prophet was impressed by Moroni with the fact that these stones were given for that very purpose. It hardly seems reasonable to suppose that the Prophet would substitute something evidently inferior under these circumstances. It may have been so, but it is so easy for a story of this kind to be circulated due to the fact that the Prophet did possess a seer stone, which he may have used for some other purposes.”
(Joseph Fielding Smith, Doctrines of Salvation, 3:225–26.)
A seer stone was INFERIOR to the Nephite interpreters so why would Joseph Smith use the seer stone when he could use the Nephite Urim and Thummim? Why would you use a fork to eat a bowl of soup if you have a spoon sitting right there?