The reason that alcohol is treated this way is not the spectacular and unique nature of alcohol as a cause of accidents. Most accidents are in fact caused by inattention due to the driver doing something else while driving or lack of sleep. The reason that alcohol is treated in this way is that people can easily imagine it being uniquely dangerous when it is not. It is clear that alcohol can impair one's driving ability, but it is just as clear that a low IQ can have the same effect.
I think Sean's article answers all of your objections, so I won't go over them in any detail here.
The key is, as Sean argues, we can respond appropriately to criminally irresponsible driving without making any particular level of blood alcohol illegal. This is the way the law is supposed to work.