Why are some denominations so against the idea of personal revelation as a source of Truth? In the spirit of On what basis do some denominations prohibit Christians from drinking alcohol? and Why do Young-Earth Creationists make such a big deal about the YEC view

First, what this question isn't:


It is not asking whether the gifts discussed are still valid today.  (That's been asked already).
It is not asking if the Ongoing Revelation as believed by some denominations is valid or true.
It's not about asking for a Scriptural reason.  As noted in the question linked to in the first bullet point, Scripture can be interpreted either way on this.


Put other way, this is not a "Truth" question.  It's a question of the cultural and doctrinal understandings that leads to adherence to a particular view.  In other words, what are some of the reasons some Christians are so wary of a view that other Christians are so willing to embrace?



Many of the Pentecostal denominations believe that things like speaking in tongues, prophecy, and ongoing revelation are still happening today as ways for God to provide us with Truth.  Likewise, the LDS Church believes in continuing revelation to the Church through the Prophets, as do others.

Some of the more conservative/Fundamentalist denominations believe that extra-Biblical revelation stopped with the closure of Canon, and tend to view modern day practice of these types of personal revelation as false, and even dangerous.  Most people can understand the Scriptural arguments, which would lead to believing that such a view is either true or false, but why dangerous?
According to the doctrines of Sola Scriptura, and Biblical Infallibility we already have completed revelation from God in the Bible, and this revelation is sufficient for us to know all that God desires of/for us: "All Scripture is God-breathed and profitable for teaching, for reproof, for correction, for training in righteousness; so that the man of God may be adequate, equipped for every good work." - 2 Timothy 3:16

The problem is that if someone says God told him something (even if it's not unbiblical), I have no real way of knowing that he is telling the truth (unless it is clearly unbiblical, in which case I know that he is either lying or deceived).  In fact, this is why there were signs and miracles at the founding of the Church (before the completion of Scripture): to demonstrate that the apostles' teaching was indeed Truth from God.