A lot of functional alcoholics can seem pretty normal at ranges from .2-.3. As a matter of fact, they seem sick and cannot function without alcohol. I've seen it personally many times with the native population. It doesn't mean we should allow these people to drive only when they are "normally drunk" simply because they have trained their body to function that way.
Also, the purpose of state tests is not to figure out exactly when a person is 'impaired' or not. That would be a nightmare for enforcement, not to mention it would present a field day for people trying to avoid conviction.
BAC is like a speed limit, you set it at a point that would be dangerous for most people to exceed. I haven't seen a lot of evidence that .05 is a good place to put that speed limit, which is why I'm reluctant to endorse it. As for other measures of impairment, it is already legal for cops to arrest you if you show signs of being impaired in ways other than alcohol.