I have an ongoing argument with an acquaintance about spelling and grammar. I am constantly, but (usually) gently, gently correcting her mistakes and she is constantly, and not so gently, calling me a Grammar Nazi and to back off. Her argument has been that, in this age of texting and 140 characters or less postings, “proper” English is no longer necessary. In fact, she believes that adhering to the rules bogs communication down.
Regardless where you stand in that argument for the majority of the population, if you are a writer of any kind you must be the best-spoken and written person that you absolutely can be. This is not a once learned and go process, either. It is part of your toolkit on which you must constantly work to improve.
Think of it this way: when you go to the clinic to get a check up, do you want a physician that doesn’t know what your humerus is? No. No, you don’t. Now, is it necessary for a physician to know the name of the bone of the upper arm? Probably not. A patient isn’t going to use it. Another doctor might, but as long as the physician in question knows what he needs to know about the arm, does the name really count?
For the love of all that’s holy, yes, he does! If that doctor is looking at my upper arm and trying to determine why it’s suddenly sprouting purple tentacles, I want him to know everything there is to know about that area of the body. You never know what will come in handy.
Similarly, being a writer means that you are the expert. Your most basic tool is the language in which you write, no matter if you write ad copy or multi-book epics. You must work on your spelling and grammar as often as you work on your storytelling. Without this skill, your writing lacks teeth.