Why do residents of Solaria bother with last names?
Every person is guaranteed to have a unique first name, so last names don't distinguish there. Also, children have no idea who their parents are and vice versa, so last names aren't useful for identifying lineage.
The only thing i can think of is that they identify spousal relationships, as a woman takes on her husband's name. But on such a small planet, and especially when even married couples hardly ever "see" each other, is that the reason?
(Side question: Being that kids don't inherit their parents' name, how do they get a last name?)
Note that this question is based only on information from The Naked Sun, not later novels.