It does sound reasonable as a conservative default, but the same could be said about driving after two or three drinks—while it certainly doesn’t make anyone a better driver, good laws often try to strike a balance between public safety and not over-criminalizing extremely common and relatively low risk behavior.

Edit: What do you think would happen if we made a strict law that anything more than 0.0% BAC = a DWI? My guess is you’d end up with a lot fewer slightly intoxicated drivers, and a lot more extremely intoxicated drivers, because people will figure they might as well just drink as much as they want if they’re already breaking the law anyway.