It's rude to start texting when holding a conversation.
John is on a date with a girl, and he gets a text from a friend.
John wants to answer his friend to make sure everything is ok.
John waits and excuses himself to the bathroom later and checks his phone then.
John takes out his phone and texts his friend.
John saves face and does not appear rude in front of his date.
John appears rude to his date, and he may not get a second date.