As a novelist, the correct usage of grammar in my prose is essential. Although the usage of grammar is a little confusing, I still enjoy studying and practicing them. I wouldn't be able to complete my novel if it weren't for the help of technical English.
I find it weird when my budding novelist told me that he doesn't like grammar. (Perhaps that's the reason why his work is always in a mess.) He seems to enjoy writing, but his style of writing is always incoherent.
Shouldn't grammar be the most important factor when writing fiction?

I agree, my friend. Poor grammar is distracting; ruins the flow of the story. Good luck, man.

Sometimes you cannot interject reality with proper grammar.

You might cal a spade a spade,
but the construction worker might call it a "f****n shovel".

Many best sellers are written by authors who appear to have no clue as to correct grammar and punctuation usage. When you reach a certain level as a writer, it seems you can break all the rules - and no one seems to care.

Obviously grammar is used so the reader will understand what you mean. They do not hear it in their head the same way the writer does without proper grammar and punctuation. However, grammar can be dropped for rhythm and effect in a novel (imho). For example John Steinbeck never heard of a period, but his novels are wonderful. The use of contractions is necessary in a novel or dialogue doesn't sound real; although you aren't allowed to use them in formal essays. Does the novel flow and make the person feel? Do they have to stop and wonder what you meant? These are the important questions more than whether or not perfect grammar is used.

Some very powerful effects can be reached through selected use of broken grammar rules. However, you must know when to break the rules and why you are doing it. Most often, it is done while trying to create a dialect in a character's speech. You won't publish any novels with poor grammar.

'Ulysses' by James Joyce would be much the poorer if he'd allowed linguistic conventions to overwhelm something more important: linguistic beauty.

"Most" important, not at all. The *most* important factor in fiction is that it should be interesting. As far as grammar goes, though, the most important thing in fiction is that your readers understand what you are trying to say. Of course, the point of good grammar is or ought to be to make ourselves understood, so there shouldn't be much conflict there.

There is a school of thought that obscurity and pointlessness are good things (e.g. Salman Rushdie), but I think that that is just a smokescreen for bad writing.

No, it should be imagination. While I agree that grammar is important, if you don't have any imagination your stories will suck.

I've worked as a novelist and as an editor at a publishing house. I hated it when we received manuscripts that were full of grammar errors. I actually had the author of a rejected manuscript tell me that there was no reason for him to write correctly because that was what copy editors were for. Nevermind that after a certain number of errors, no matter how compelling the plot, I would reject the work out of hand.

If you can't structure a sentence, odds are you can't structure a novel.