Facebook was made large by the fact that you can find people you used to know, or haven't spoken to in a while, or aren't that close to. You can find anyone, it's easy. Removing the real names policy does away with that, and I would say seriously threaten's facebook's business/growth/future. If you don't know your old girlfriend or your cousin or your highschool bandmate's username, how are you going to get in touch…
First, the most common use of a "chosen name" policy would be to let you find someone by the name they actually use - stage name, pen name, drag name, or unofficially changed name. Second, the less common but equally important use of a chosen name policy would be to make it hard to find someone who doesn't want to be found eg: by an abusive, dangerous ex.
I see countless individuals making their Facebook profiles entirely public, and it's quite frankly scary. At that point they have no control over what the internet sees of them. You can't even change it after the fact, as it's probably been indexed in search engines and cached in internet archives.