Very interesting thatbags. As often happens with me these days, I find it difficult to come down on one side or the other.
In a way, I think it's important that people know the rules because as the "pedant" said, some people set great store by them and to break them when, say, applying for a job could well scupper someone's chances.
On the other hand, I also agree that grammatical rules and certain modes of speech, vocabulary, etc., have no intrinsic value but have been used to maintain the power of a small elite over the majority by creating a sort of "U" and "non-U" climate in speech conventions.
I think what the "pedant" was trying to say was that it is easy for someone who is already a successful and respected writer to flout the rules but it may be unwise for other people to do so.
I agree with the other chap who pointed out that certain rules, whilst being technically correct, are no longer in common usage and people who rigidly stick to these outdated rules will not necessarily be perceived as good communicators.