"Never pull out your phone unless you politely excuse yourself from the people you are with."

Among people I know, that is the standard practice; it's not something I've ever heard anyone advocate, it's just what everyone does. I'm intrigued as to what context you (and, by the looks of the comments here, lots of other HN posters) are in where people just break off in the middle of social interactions to mess about with their phones. That's such a basic violation of (what I take to be) standard interpersonal norms I have trouble imagining anyone doing it.