Being able to search and find people by the names they actually use on a day to day basis is actually a feature I value very highly in a social network. Being able to hide from abusive people in one's past is actually a feature I value very highly in a social network. I know a number of people -- transpeople, people from other countries, people who just don't like their given name -- who don't use their "real" name in "real" life. Why should facebook require one's legal name when no in person congregation requires you to wear a name badge that identifies you with a name you may not be comfortable with?
If you don't like Facebook's value proposition, you're always free to build your own social network with the features you like. Nobody forces you to use Facebook.
You may not be aware that in some jurisdictions (Quebec is an example), marriage is not a sufficient legal reason to change your legal name, yet most people informally identify with a spousal name change. It would seem like an overstep for Facebook to regulate against this by policy. That's only one of many, many examples where people shouldn't be forced into using their legal name simply BECAUSE it's their legal name.
What is so important about displaying the legal name, especially in cases (like the GP explained) where nearly no-one in ones social environment knows it? If Facebook had legal reasons to know it they still could make displaying it optional.
