As Clevernick mentioned, English grammar actually makes a scary amount of sense, but is based hugely upon word order. "The man eats fish." "The fish eats man." This is not so true in many other languages; English allows brevity and inflection, and avoids a long series of conjugations, but it also means screwing up sentence structure can render a statement unintelligible. (As a sidenote, most English spelling makes a great deal of sense if only you were speaking the Middle English original. A knight, for instance, has all his letters pronounced).

It's interesting to note that most bad written grammar is glaring for the simple fact that it's so different from common usage. Grammar is the correct way to speak, not artistic interpretation. For the most part, correct grammar sounds more natural and coherent, because that is the point of grammar.

There are exceptions, and I don't think dialogue or fame have much to do with them: I wouldn't recommend anyone say "Is it I, or does this sound funny?" even though that's grammatically correct. Famous writers break convention because they understand the impact of doing so, and use that as a form of communication, not because they are above it (as a good photographer will use black and white because it enhances aspects they wish to communicate, not because they're too good to bother with proper film). Dialogue is the same way; if I use "ain't" in a dialogue, for instance, it's good to understand that my character will come across as uneducated, unintelligent and rural. If I intend that, then it enhances my work and is proper writing; if it doesn't, then it is failure. I need to understand that in writing because accidentally having a slick world-famous lawyer spout "ain't" can destroy an entire piece.

Of course you can break rules without knowing them and get away with it, but that's simple luck, which isn't very reliable. And frankly, if you don't know what's right, you'll be the last one to know it's wrong, and probably the last one to realize it didn't work at all.

As for authorities, we have whole institutions of learning at our disposal, and it's worth our time to consider what they're saying. Resources compiled by professionals and professors -- such as the MLA handbook -- are subject to extensive and intense peer review, and give us a communal assertation of what is correct. Nor are the rules of the English language vague: what you and I speak and write every day is not a simple circumstance, but the work of generations to refine and define. Thank or blame the Victorians, they've had their way and defined our language with rigid rules of standardization, and society has incorporated these eagerly. So in short -- assuming there's no authority is really just trying to impose your own. Not a horrible thing (I do it too; I refuse to capitalize god or the pope, or christianity or islam. I do enjoy capitalizing in Emphasis, a custom the Victorians ended for Reasons I Disagree With). But again, it's important to understand that such a thing is unusual, and to consider my audience. A sci fi novel about space exploration is not a proper forum to introduce unique capitalization; an essay on the benefit of grammar might be.

Anyhow; thank you very much for your post, Clevernick. I agree with most, though not all; but it's certainly all worth learning.