Technically perfect writing is important when it comes to journalism or nonfiction, and especially helpful when writing with short deadlines. Fiction writing is different though. Nicole Bernier, over at Beyond the Margins, explains why grammatically sloppy writing might be the product of greater creativity:

Sometimes when creative writers say they don’t notice their own typos, it has a whiff of, well, humblebraggery. They’re on a roll, blinded by their vision — the fingers cannot be stopped, the whole brain engaged in its art. That’s why God invented proofreaders, right? To straighten up the mess of the visionaries. The people who sweep up the mistakes are valued professionals, to be sure, but they are not the artists. They are the people with the golden dustpan and brush.