Speaking proper English is always useful. If one is in academia, if one aspires to politics, if one wants to convey linguistic agility, the ability to speak the Queen's English is important. It promotes an image equal to that of being well-dressed or driving an expensive car. It suggests learning, status, money, and the affectations of the upper classes.
In a word, superiority.
Regardless of the language, the ability to speak it properly conveys an image of scholarship and-or intellect that is above average. Those who speak properly are generally accorded more attention and more respect in certain circles . . . but not all circles.
It's not limited to English. There are at least 750 languages spoken on our little planet, though English, after centuries of institutional demagoguery, has become the world's "business" language and we who speak it are damned lucky for it.
Whether we speak it well and how we're able to modify our speech for our audience has consequences.
That's pretty complicated, so, let's stick with American English. Americans have evolved and modified English considerably. The Australians have contributed to the evolution as well. In fact, every culture that has adopted English (India, for example) brings new idioms and constructions to the plate. Language evolves. Chaucer's English is not the same as Shakespeare's which is not the same as Robert Frost's. And the English spoken by Barack Obama and John McCain isn't the same as spoken by rappers 50-Cent and Eminem. They each appeal to different audiences for different goals.
I grew up speaking English and my mother made sure I could read and write it as well. Later I learned Italian, German and Spanish in our travels. What I learned from that was that speaking a language properly didn't always make me popular, didn't always keep me safe, didn't always help me make my point. I learned, instead, to modify my language, my syntax, and my accent depending on my audience and whether I wanted to convey my ideas rather than flaunt my linguistic acumen. Believe me, orating like Abraham Lincoln doesn't go far in a pub in Queens.
The most useful talent is to know language well enough to modify our use to meet the standards of our audience. Politicians understand this more than any of us. To "speak" over the heads of listeners is to lose them. In some cases, it can inspire contempt and jealousy that works against the best intentions.
I once sat at lunch in the happy company of accomplished peers from Maine, Texas, New York City, and Atlanta and found myself in the unenviable position of translating the comments of one speaker to the rest of the group. For some reason, perhaps because of my experience with different cultures and languages, I was the only one who understood the slang and dialect of what any single person said.
I'll never forget it. I'd never anticipated being in such a situation. It made me conscious of how speaking some vague ideal of a language was secondary to being able to really communicate with others.
I personally adhere to speaking properly and correct my children when they slip into the vernacular of their generation and say "me and her are going" instead of "she and I" are going. I wrestle with them over the mysteries of when to use "I" or "me" in a sentence. The answer to the question, "Who is it?" is, "It is I," but who says that? We say, "It's me." The proper English answer to the question "Are you (so and so)?" is "Yes, I am he (or she)," but who says anything but, "Yeah, that's me." Such fundamentals don't even touch the surface of distortions of double-negatives (I didn't do nothin'), which technically implies that the speaker did "everything", or the colorful use of conjunctions such as "she don't" or "whyn't you" instead of "she doesn't" and "why don't you."
Learning language is fundamental. Learning proper use of language is an advantage for social distinction and advancement in the workplace. Knowing a language well enough to adjust to your audience, your aspirations, your friends and your general well-being is a personal characteristic that transcends word skills and allows you to really "communicate."
Do we want to be known for our perfect syntax or for our good ideas? Communication is everything. A great idea is more important than the words in which it is expressed. Beware though, that great ideas are often ignored until someone who speaks well steals them.