First of all, this assumption overrides what the writer of Hebrews says in Heb. 1:1-2: "God...in times PAST" spoke "by the prophets [plural] "Hath in THESE LAST DAYS spoken unto us by His Son..."
Jesus Christ is our Living Prophet, the Full Revelation of God.
Secondly, it is outlandish that LDS prophets presume that God only and always speaks through them. For example, at the 2/26/80 BYU Devotional Assembly presented 14 "Fundamentals" in following The Prophets. Of course, #1 was: "The Prophet is the Only Man who speaks for God in everything."
How arrogant. How parochial. How limited its view of how God speaks.
LDS often "get on the case" for Christians for not recognizing the "need" for fresh, living revelation today. They think Christians "shut up" heaven.
In fact, it's LDS who "shut up" heaven, as if the revelation of God poured forth through one mere cloud that hangs over Salt Lake City 24/7. (But somehow this rain, if it really ever falls, hardly ever gets placed into the D&C scripture reservoir]