Michael Horton rightly disagrees with Wayne Grudem on the definition of prophecy:
Grudem believes that the kind of prophecy that is ongoing in the church is distinguished from preaching and teaching by being “a spontaneous ‘revelation’ from God…In my view, this interpretation introduces a definition of prophecy that is not consistent with its practice in the apostolic church. Nowhere is prophecy distinguished by its spontaneous quality.
What he rightly disagrees with is a flimsy distinction between preaching and prophecy. Surely there is more to the distinction than that.
However, I am guessing that Horton disagrees with the “spontaneous” factor because of present day practices in Charismatic circles. I cannot prove this; I just think this is so. I do think prophecy in the bible is spontaneous (a prophet receives a word from the Lord instantaneously). Every time God speaks to a Prophet, the Prophet was not pondering on what God might say. God just said it.
Perhaps the distinction between preaching and prophecy is better explained to be like the difference between a miraculous healing and ordinary surgery/medicine. I, personally, do not desire one over the other. God accomplishes both. One immediately, one mediately. When I pray for healing, I expect God to do either one. Is there something wrong with me for not desiring one over the other, but being equally happy that God is glorified in both? Same with preaching and prophecy.
Why do I believe prophecy could exist today? Because the bible does not tell us it would cease. Rather, it should be earnestly desired (1 Cor 14.1), it is speaking to people for upbuilding, encouragement, and consolation (1 Cor 14.2), and it builds up the whole church (1 Cor 14.5).
Why do I believe it is infallible? Because every single explicit example of prophecy is a direct word from the Lord, and God is infallible.
Why do I believe prophecy could be infallible and, yet, not be adding to Scripture? Because I think that is the nature of prophecy throughout the Bible. Next time, I will give some examples to prove that.