In the adult world, it is absolutely essential. In fact, I think is the most important tool for making a good impression. In a job interview, shopping for a car, getting employee assisstance in a store, etc., nothing commands respect like a strong vocabulary. Even if you happen to be wearing a $500 suit or dress, you'll see the change in stranger's demeanor if they peg you for a twit.
Take it from an old guy, Hypsters, proper grammar is your friend. Read those books and arm yourself! Save the hipster slang and colloquialisms for your buddies and/or family.