I don't think that the most salient point of the letter from Desiré was about denying prophecy & vision, but more so the statement:

"You say that's not in the scriptures? Well, a writer writes a sequel. God does too."

If that is what some people may believe, then that is troubling. Theoretically, a person could claim radically altered doctrine in the form of a prophetic utterance, claiming "God's sequel" to the scriptures. I that is accepted, then it closes the door to using the scriptures as the ultimate standard to compare the new doctrine against. People would have to accept the new & different doctrine as "God's sequel" to the scriptures, under fear of rejecting God's new prophet. Then if like Pharaoh's magicians, the speaker imitates some of the gifts of the Spirit, the hook is set.

I think it is a dangerous precedence, if anyone thinks that they can claim that they have a sequel to the word of God, from Joseph Smith to posters on YouTube.

Gino