Mr. Hall expounds a couple of key thesis. First, culture is not just the medium of communication. It is a method of communication all on its own. Second, if one "maps" cultural characteristics in ten "primary messaging systems", one can gain insight into the formal, informal, and technical aspects of that culture. Those ten primary messaging systems are: (1) Interaction, (2) association, (3) subsistence, (4) bisexuality, (5) territoriality, (6) temporality, (7) learning, (8) play, (9) defense, and (10) exploitation.
This book is indeed somewhat dated, but one can certainly see that the use of this monograph is that it provides a systematic way of analyzing culture. This can be useful when traveling or working abroad. Likewise, one could use these templates to improve ones understanding of his own culture. It is this latter purpose that makes this book relevant today.
The Silent Language is short enough to be easily read. However, for the sake of brevity, it sacrifices a more systemic or detailed analysis. As such, the author resorts to a limited amount of anecdotal information to support his framework. Anecdotal information is useful, but some more statistical analysis might be worthwhile. Indeed, one could argue that defining each primary messaging system as a collection of "sets", it would be possible to apply set theory from mathematics to anthropology.
While I have trouble believing that culture is a form of communication vice a medium of communication, I will probably use Mr. Hall's framework to analyze the "culture" each job and workplace that I deal with in the future. Likewise, I think anyone who works in management or leadership jobs would find this book useful.