After reading the following links my answer to Sally would be that the problem is exactly the opposite of the implication in the question.
Body language is used by the instructor to pass messages or to receive messages from students.
Body language in itself does not help students learn and can hinder if the instructor concentrates too much on his or her own presence. One reason that TV never fulfilled some people's hopes as a mass educational tool was the lack of interaction. We tend to increase learning when we increase student involvement which can correlate with increased interaction - hence the promise of the internet. Even simple text-based interaction can involve students as it removes barriers to their interaction. Instructors are sometimes overwhelmed by the student interaction via email.