In the book Kay has to persuade Michael to do something about his face because he looked like a gangster. If she hadn't done so, she would have been reminded every morning about her husband's occupation.
Good point Danito,
I believe Puzo (and later FFC) included and played up the lasting damage of the blow to Michael's face in order to illustrate Kay's desire to reject what Michael was/ was becoming. By having Kay insist that he get the surgery, she didn't have to live with the day to day reminder that her husband was a gangster.
