On one hand, being a drunk driver has never killed a single person if no other laws were broken. It's like speeding. No one has ever been killed by a speeding car that did not also break another law. These two factors simply increase the chances that another law will be broken, and increase the chances of a more negative outcome.
On the other hand, Even having 1-2 beers with a meal can reduce your reaction time by 1/2 second. These are huge factors when it comes to accident avoidance.
You know, it's kind of ironic. I am an amateur racecar driver. You are in a controlled environment, where everyone is going the same direction, with thousands of dollars of safety equipment, travelling at almost identical speeds - and there is zero tolerance for drugs or alcohol. Yet, we put our kids into a car with minimal crash protection, travel on roads with people going opposite and perpendicular to us, going all different speeds - and allow drivers to be "only slightly" impared.