Married couples in Japan are required to have the same surname, but the law may be changing soon. I'm in favor of people being able to name themselves whatever they want, but I think it's a mistake to mock the arguments being made in Japan for the status quo.
But some lawmakers of the ruling Liberal Democratic Party voiced opposition to the proposal, arguing the new system would cause the collapse of the family and undermine the sense of unity among family members.
"Collapse" might be a little strong, but there's no doubt in my mind that having different names can undermine family unity. You may believe that the effect is minor or that the matter shouldn't be of concern to the government, but I don't believe that the argument for family unity is worthy of mockery. People should have the freedom to change their names, but children in families with multiple last names will certainly be affected to some non-zero degree.
Surnames are an important part of identity, which is why people should be free to change them but is also why a person should weigh any decision about his name very carefully. Children especially build a sense of themselves and their family from the name they share, and a child with a different name will certainly feel like an outsider rather than an integrated part of a greater whole.
(HT: Paul Hsieh.)