I always emphasize good English composition and here is a nice excerpt about it.
No matter what the situation or setting, the words you choose say something about you. Whether in casual conversation with a new acquaintance or in a cover letter to a prospective employer, correct English grammar conveys education and intelligence. It says you have the ability to express yourself clearly. When you speak, others will understand you and will pay attention to the content, rather than being jarred and distracted by poor grammar. When you write, others will take pleasure in the clarity of what you've written. You may worry that using correct grammar will make you sound snobby and set you apart from your friends. But it isn't necessary to communicate in a stilted, formal manner or pepper your speech with so-called ten-dollar words that require a dictionary to decipher. Just say what you want to say, but say it correctly.
Check the entire article on this link: http://grammar-and-composition.yoexpert.com/english-grammar/why-should-i-bother-using-correct-english-grammar-32216.html
