Just a note from me - I also agree this thread evolved (or rather devolved) in a rather unproductive and strange way.
One important note that came out, though, is that adding a new client message does have a backwards compatibility issue - intelligent proxies such as pgbouncer/pgpool will probably break once they see an unknown client message. Even if they don't, they may miss potentially important information being transmitted to the server. Whether this is a deal-breaker for introducing new messages is another matter (I personally don't think so).