Here are Busenitz's 5 dangers:
1. By creating a category of modern “prophecy” that can include erroneous messages, this view makes it unnecessarily difficult for the church today to identify and refute false prophets (cf. Matt. 7:15). It further neuters (i.e. ignores) the strict requirements on true prophecy found in Deuteronomy 13 and 18.
2. By defining prophecy in terms of impressions and subjective guidance, this view provides no objective or authoritative means by which a person can know for sure if a feeling is from God or some other source. It also provides no objective or authoritative means by which church leaders can evaluate for sure whether a “prophet’s” message is legitimate.
3. By teaching that God still gives revelation today, this view encourages believers to look for messages from God outside of the Bible. While continuationists insist on a closed canon (and rightly so), this view of prophecy — in practice — calls into question the sufficiency of Scripture at the most practical levels of daily living.
4. By using terms like “prophecy,” “revelation,” and “a word from the Lord,” this view has the potential to manipulate people by binding their consciences to a fallible message or compelling them to make unwise decisions. Though proponents insists that congregational prophecy is not authoritative (at least, not at the corporate level), their understanding of prophecy is highly vulnerable to being abused within the local congregation.
5. By allowing for error in prophecy, this view permits people to say, “Thus says the Lord” when in fact their messages are fallible and erroneous. In effect, it allows people to attribute to the God of Truth messages that are errant, which is a very dangerous thing to do. Furthermore, by redefining fallible messages as “prophecy,” it demeans and cheapens the true gift of infallible prophecy as it operated in the Old and New Testaments.Read the whole post for the context and some good discussion of Grudem's specific claims.