Good grammar along with good spelling are simply tools, with some written work it is absolutely essential yet with others it's not. Legibility and understanding the meaning of what is written is the most important part of any piece work, if you do that you are onto a winner in my book as all too often self appointed grammar police can't wait to rip someones work apart yet never read it or enjoy it's message. Look back far enough to see where language came from and how it evolved, there is little to get excited about grammar. Good communication and understanding is more important than grammatical correctness. From what I have read even some of the best get it wrong so why ruin a good read trying too hard to make it perfect. So in answer to OP...No it's not essential but the prospect of the other extreme is probably harder to endure.
