When a writer sits down to compose a sentence, he has an idea in his head that he wishes to see reconstructed in the mind of another. This is the ultimate purpose of writing: to move an idea from one brain to another. Think of writing as a low tech version of the transporter machine on Star Trek -- a process that deconstructs an idea in one brain and reconstructs it in another.
People living the same time, place, and culture often think so much alike about some things that only a few words are required to pass an idea from one head to another. For example, all I have to do is say the words "Santa Claus" to efficiently transport a complex image complete with beard, red and white coat, and bag of toys from my mind to yours.
Unfortunately, the ease with which ideas like "Santa Claus" or "baseball" or "the Beatles" allow us to transfer concepts also tempts us into laziness. Ideas like these are the exception and not the rule. If I tell you that Xa inhibitors are the wave of the future for improving hospital length-of-stay for atrial fibrillation admissions, you probably need to know more. No one is a mindreader, and most complicated ideas require effort to convey.
Precision is what we want here. The reader wants to know exactly what the writer is thinking. The reader may not agree with the idea or find it pleasant, but she still wants to know what the original idea was, with as much clarity as possible. This is the point of communication.
Since a writer can't directly transport an idea from his head to another like an transporter beam flashing between two starships, the only way to exchange ideas is to have a set of agreed-upon tools that both the reader and writer understand well. When the writer makes proper use of these tools, the reader, understanding their meaning and purpose, can accurately reconstruct the idea in her own mind. The new idea in the reader's mind will be a mirror of the writer's idea, rather than an interpretation, or worse, a best-stab guess.
This process only works if there are strict conventions. When the writer capitalizes a word, the reader knows why. When the writer uses a comma or a semicolon or a period, the reader understands the need for it. The number of tools does not have be vast. Only a few will do, but the few that are used must be precise. The more precise the tools, the more exact the communication. More tools, such as a larger vocabulary or more sophisticated grammatical conventions (like the subjunctive voice or complex sentence structure), can be helpful because they provide more bandwidth, but what really matters is that the ones that are used are understood by both the writer and the reader. If I am trying to buy 10 pounds of apples from you and my pound is an ounce less than your pound, we will never reach an agreement on quantity or price or anything else. We may end up in a fight.
This is why picky readers are right to ignore writing with poor grammar, punctuation, or spelling. The picky -- or should I say discerning -- reader knows that if the writer cannot use his tools with precision and knowledge, he probably is not capable of fully transmitting an idea. What the writer intends and the reader receives could be different. The intent of the writing is defeated.
If I open a bottle of milk and it smells sour, I do not drink it. The foul smell indicates the milk will not provide the benefit intended, nutrition.
So it is with writing. If a piece of writing is full of grammatical and punctuation errors, this indicates that the writer is not in command of his tools and is probably not capable of providing the benefit intended, the recreation of an idea in the mind of another.
If that is being arrogant or picky, go drink your own sour milk. As for me, I don't touch the stuff.
What about the great writers, some may object -- the Joyces, the Faulkners, the Porters, the Woolfs -- who break the rules? Are they bad writers? Why do they get to break rules that ordinary folks can't?
The answer is, they cannot break the rules any more than we can. What they can do is play off the rules, which is an entirely different thing. I know James Joyce can write perfectly correct English; I know this by looking at more conventional writing he has done, and I know this because if I read his writing closely enough I can see that there is a convincing logical system present. Joyce doesn't break rules, he plays off of them, challenging our perceptions by writing in a way that contrasts what we expect with what is on the paper.
When Emily Dickinson ends a stanza without a period, we know she could have used one, and that she knew one belonged there. The absence of the period is Dickinson's statement -- we know it is supposed to be there and she does, too. That shared knowledge becomes the precise tool of communication.
The problem comes with a writer who has not yet earned that kind of trust from the reader. Does the writer know a period belongs here? Does he know the difference between there, their, and they're? If the reader cannot be certain, precision is lost, and along with it communication.
This is the difference between bad writers who break rules and good writers who play off them. A good writer demonstrates through her writing that she can be trusted, that when she seems to ignore the rules, she is really playing off of them. A bad writer cannot do that because the reader can't trust this is going on.
How do you become a trusted writer who can play off the rules? By writing correctly and intelligently. Once you establish credibility with your readers, then you are allowed to make grammatical and punctuational mistakes