Whoa whoa whoa. WAY off the mark. Mental impairment can begin as low as .02% BAC, regardless of how different a person may think when he/she is at .02, .04, .08, or any other number. It is the result of the breakdown of alcohol by the body and cannot be considered negligible even with the consumption of a single beverage. .08 is considered the legal limit because regardless of the many factors (such as weight, tolerance, genetics, etc), for the majority of individuals .08 is a considerable impairment, and while many people may be able to drive by focusing REALLY hard on the road, their faculties are quite hindered.
Impaired is impaired. If someone smokes pot and gets busted, they get locked up for driving under the influence, just as an individual who is drunk driving. Different drugs, same result. You cannot take two mind altering drugs and say "Oh, this is safer because I can take a smaller dose and it won't affect me one bit". Why not? Because it's a lie, pure and simple. The drug has an effect, and to claim any different is simply naive.