Yes, there are excesses among the Charismatics but the excess is not really an exception because it is so common that it is present in nearly every meeting and nearly every practitioner engages in it at least once in a while. It is not only speaking in gibberish but also being 'slain in the Spirit' and 'holy laughter' and the more recent jerking and twitching as well as the short lived claims about glitter and gold dust falling from heaven in meetings. Huge numbers accept at least parts of the health & wealth message. The movement is so full of excess and abuses that one wonders why it is popular.
This is where balance is crucial. Yes, there is a lot of excess within the charismatic movement and when you get into the hypercharismatic crowd they do all sorts of stupid stuff that bears virtually no resemblance to Scripture and yet they insist it is all of God because "it's anointed". But that doesn't mean that there isn't a lot of genuine Godliness within the charismatic movement.
When Paul wrote "test all things" it made it clear that among the excess there is good stuff. If nothing were good there would be nothing to test - it would be as simple as "tongues - bad / prophecy - bad / healing - bad" and so on.
And the 'prophets' in the Charismatic movement do no better; when one takes the time to check what is said by these alleged prophets and test it against holy scripture and against history the vast majority of them fail and fail so badly that it is a wonder that they are listened to by anybody. Some among their number openly say that only a low percentage, 10% or less, of the messages are genuinely prophetic and that even among the famous and most reliable they say that around 20% to 40% of what is said is not genuine. But even 1 non genuine prophetic utterance from one who claims to be a prophet is sufficient to undermine the claim entirely. Who would accept Jesus as the Messiah and Lord if 20% of his prophetic utterances were not genuine? Surely it is impossible to accept that God is author of these utterances when some proportion (no matter how small) is not from God. Even Saint Paul, when speaking his own views and opinion in holy scripture is careful to say that's what he is doing and not to falsely attribute his personal views to God.
This is where we need to consider prophecy in the OT and NT contexts.
In the OT a prophet spoke for God and if he got it wrong he was executed. Even then there were some cases where prophecies never came to pass - the destruction of Ninevah prophesied by Jonah never happened, Isaiah's prophecy of the virgin conceiving the child named Immanuel wasn't fulfilled for hundreds of years, and some of what Daniel prophesied still has yet to be fulfilled. Of course Isaiah and Daniel established a track record rather than just spouting off a load of vague stuff in the hope some of it would come to pass, and of course since the entire city of Ninevah repented it's not such a far-fetched call to say that Jonah's prophecy served its purpose.
In the NT we see Paul saying "let two or three prophets speak and let the others judge". There would be no need to judge if prophets in the NT context were supposed to be infallible, although even there it comes down to how we present a prophetic word. The kind of person who speaks in King James English, proclaiming in a booming voice "Thus saith the Lord" isn't showing a whole lot of humility, and if they get it wrong they need to be reined in one way or another. The kind of person who approaches someone in humility and says "I think God is saying that..." is another matter entirely - they are merely presenting what they believe God to be saying and whoever receives the word can decide for themselves whether it was the result of God speaking or the person eating too much cheese before bed. If such a person gets it wrong once in a while it doesn't warrant the outright rejection of everything they say from then on.
Some web sites out there (The Elijah List comes to mind, I'm sure there are others) present a daily stream of drivel presented as prophetic, usually couched in very spiritual sounding terms (most of their "prophets" seem to be taken up into heaven to receive a message, or woken in the night by an angel who gave them a message, or something equally unusual) that are generally either so vague they could apply to just about anything or specific enough to be tested and found not to have come to pass.
There's probably another Poor Theology thread in there somewhere.