The implication of this passage is that the prophecy itself comes by inspiration of the Holy Spirit--it is not strictly conjured up in the mind of the prophet, interpreting events in history out of his own independent thoughts. God Himself is showing the prophet what He is saying and what the state of affairs is or will be.
I've heard it argued, when someone wants his own interpretation or position to stand without criticism, that his own view is "inspired, while others are interpreting a biblical view "out of their own mind." This is not the object of the verse, to allow someone to assert his own view while discounting all questions about the validity of his view.
The whole idea is to examine a word from a prophet or teacher to see if indeed it is divinely inspired. And that requires an examination of the Spirit, the character of God being represented, and the reality presented in the context of the moral matters being weighed.
We cannot just say, "I'm inspired by God, and you cannot use your own mind to question my inspiration!" To receive something "by revelation" is not to divorce one's self from the mind and all reasoning. On the contrary, what God truly says does reflect something that can be and should be judged.
That being said, I've had real trouble discerning many of the prophets I've heard within my own Pentecostal denomination. It does not seem in character with God for Him to say things that do not prove to be correct, obviously. But it's difficult to judge the motives of the prophet, who is a Christian, who says things that appear to be inspired, and yet presents information that does not prove to be true in the end.
I cannot judge them to be non-Christian, but I can judge their "prophecies" to have been false.
