I can't find the source where I recall reading it, but I believe the average drunk driver who causes an accident has 0.17 BAC; how does reducing the legal limit stop those who are already way beyond it?
It's like, oh, I don't know, limiting firearm magazine capacity to 7 because a murderer fired a couple hundred rounds.
That figure has remained constant, from the days when the legal limit in many states was 0.15, when they all changed to 0.10, and and now that the feds forced them to lower it to 0.08 BAC.
(Some stranger on Facebook recalled the same 0.17 BAC figure, but he couldn't find it either. I think Balko wrote an article about it when he was with CATO.)