Even in New Testament times, there were so many Marys and Jesuses around that they had to be given extra identifiers - this is why Jesus is so often referred to in the Gospels as "Jesus of Nazareth", and why we have "Mary the mother of Jesus" and "Mary Magdalene". Eventually these evolved into surnames. What's so terrible about having a surname? It's only to reduce confusion!
And as for the titles "Mr", "Mrs" and "Dr", that's just politeness in our culture when addressing someone you don't know very well. That's why your friends and family don't use them!
So basically your priorities for personating a legal name is because 1. you would otherwise feel confused and 2. because it is polite in your culture to do so?