I think that's very unlikely to happen. People who hadn't been abused would use it widely. Facebook, like Google, wants you to spend as much time as possible on Facebook, which means moving as much as your life into Facebook as possible and making things about others public by default, because that translates to more pageviews. Facebook makes money selling ads, so the more pages you view the more ads they sell. This is their principal motivation.
Keeping things locked-down goes against both precedent that Facebook itself has set and the easy way to get more pageviews. There is probably a market for such a locked-down social network for people in witness protection or secret agents, but I don't think Facebook is very interested in the space.
Facebook wants to be around for a long time. Facebook wants people to get value out of their site. This isn't a get-rich-quick scheme where the objective is to strip-mine their users. A necessary condition for Facebook making any money is that people enjoy their service. If they don't implement it, it's because they don't think it'd be valuable to enough people to be worth the opportunity cost of having people implement it, not because they think it would hurt ad revenue.
It seems to be a popular sentiment that any ad-supported company hates their users, and I don't get it. It's true for spam but it isn't true for companies that want to be around for more than a year.
The point isn't to allow her to also use the site (which would be alright). It's to make it so the site doesn't inadvertently disclose her location (as google buzz did for instance when it first popped up).
Giving people control over when they are tagged is really the issue here (and is why my comment was posted in response to that).
Keeping things locked-down goes against both precedent that Facebook itself has set and the easy way to get more pageviews. There is probably a market for such a locked-down social network for people in witness protection or secret agents, but I don't think Facebook is very interested in the space.