I dislike it particularly because of the implication of stupidity: "I believe only stupid people won't know this, but I'm going to tell you anyway". In general

I dislike it particularly because of the implication of stupidity: "I believe only stupid people won't know this, but I'm going to tell you anyway". In general it's poor form to assume readers know anything - consider (for example) a future lay-reader who is trying to find out this information.