The point of creating and using grammar rules is to facilitate communication - to avoid being misunderstood. For example, to say, "I do not want a hamburger" does NOT mean that I want to avoid a hamburger; it merely means that I have no desire to possess one - I do not WANT one, but I would accept one. However, to say, "I want to not have a hamburger" means that I wish to avoid hamburger possession. I am a substitute teacher, and I hear sloppy statements all the time from teachers and students alike; these speakers run the risk of being misunderstood. If I were in a spaceship and was receiving instructions from NASA, I would hope the speaker on Earth would adhere to my standards, regardless of what is common vernacular.