Hello everyone!
I just joined the forum after reading this discussion, and I know I might be a little late, but I was re-watching all the seasons again and came across the scene where Mycroft mentions 1972.

The way I understood it was - 'The Diogenes Club' number one rule is that there is no talking. When politicians / diplomats / heads of societies etc... get together and start talking... that's when ideas start brewing - ideas for a revolution, an uprising, a coup, even worse – murder, assassinations, etc... Since 1972 is deemed as one of the worst years for the United Kingdom as a whole, with so many strikes, accidents, deaths and general turmoil, what Mycroft seems to be saying is "It is better if politicians don't talk to each other", thus convincing John of the Diogenes Club rule of silence (and explaining why the security had to manhandle John to stop him from talking).

I hope this makes sense!