I wouldn't assume that it's so easy for Facebook to make fundamental changes to their friend system. It's woven into every other part of the site. The whole visibility thing would probably be a nightmare to implement.
Not only would it actually be challenging, but more likely than not Facebook would actually get hate for breaking the Facebook, just like every other time they've implemented any change more visible than an extra link somewhere.
I wouldn't assume that it's so easy for Facebook to make fundamental changes to their friend system. It's woven into every other part of the site. The whole visibility thing would probably be a nightmare to implement.