I have just finished a post on vocabulary and how it can contribute to clearer communication. Part of the problem with using the "wrong" word is the grating feeling the person on the other end of the communication feels. That twinge of "huh?" And that in itself will interrupt the flow of communication. Of course, this issue goes much deeper than that, in that choosing the incorrect or inappropriate word can go much further and lead to complete misunderstanding, because the word will give an entirely different meaning. But when it comes to spelling and grammar, the issue changes. I still get the "Huh?" twinge when I see something I learned was not right, and yet in today's world of relaxed rules and anything goes communication, I'm not so sure about the impact on meaning. Not so strong as using the wrong vocabulary, I don't think.