"completely incorrect" is useless prescriptivism. They can call it incorrect all they like, it won't stop anyone.
What is kind of useless is a word that can mean some thing or its opposite, and there is no way to tell from the context, eg:
"When the tiger went "Rawr! Rawrrr!", I literally fouled my breeches!"
Can't have a factual should. Shoulds are normative.
>What is kind of useless is a word that can mean some thing or its opposite
Is the word really useless or indeed useful if no one can provide a 'real' example of actual ambiguity between the two meanings? Furthermore, it's not as if ambiguity between one meaning and an opposite meaning is the greatest ambiguity there is-- if anything, it makes it particularly obvious which meaning the speaker intends.
Also, Wikipedia refers to two types of dictionary: prescriptive and descriptive. But even the descriptive type should provide information on how the word is used, eg: "sometimes incorrectly used to mean 'figuratively'".
Wait, think a minute. That description exactly corresponds with what I said. The "correct" usage of a word is precisely, exactly what the speaker thinks it means, at that moment in time.
If a dictionary listed a definition that was not in use by anyone, that would be different -- that would be incorrect.
> Also, Wikipedia refers to two types of dictionary: prescriptive and descriptive.
Yes, and there are no prescriptive dictionaries, at least not in English. The French have one, created by the Académie française:
http://en.wikipedia.org/wiki/Acad%C3%A9mie_fran%C3%A7aise
Apart from the French example, it seems telling people how to use words has fallen out of fashion, giving way to another example of evolution by natural selection.