Was Plato ever told that? Was the Buddha ever told that? Where their jobs not to philosophize? Where would humanity be if people like them didn't hang out with their pals ("disciples") and talked about work?
It just dawned on me. While I do agree that people outside of the subject might not want to hear about it while in a social situation, this popular "rule" is a hint of what turns people into drones: people that go to work thinking "it's just work" so they can go home and "live" and spend the money they've earned.
I've always said, before getting hired for the first time, that I wouldn't work. I'd get paid for doing what I like. So far, so good. In fact, I like it so much, that I won't shut up about the projects I have in mind.
Good work is characterized by quality. Quality is exuded by people who care about what they do. You can't care just nine-to-five, you either care or you don't. This is why creativity and all the cognitive process behind good ideas can't be bound to a particular time. Good ideas suddenly hit you, and they have to mature and be thought through.
Now I'm going to appeal to my vast audience:
People (person?): talk about work outside of work. Think important things. Talk about things that serve a purpose, that sparks creative thought on other people.