Is there nothing more rude than dropping by to criticize someone else conversation; not to join in but to try to demonstrate your own "superiority" by saying that the participants are inferior for even having the conversation?

There are a billion things to talk about. If you think something else is more important then by all means talk about it.   

If you want to talk about 'Guernica' or 50 years passed classroom bigotry, no one is preventing you from buggering off and starting your own thread. But butting into a conversation that you say is beneath you simply to say that it is beneath you, is really nothing more than demonstrating your own ignorance.