Research undertaken within Europe has indicated that pilots have shown impairment in their ability to fly an ILS approach or to fly IFR, and even to perform routine VFR flight tasks while under the influence of alcohol, regardless of individual flying experience. The number of serious errors committed by pilots dramatically increases at or above concentrations of 0.04% blood alcohol. Some studies have shown decrements in pilot performance with blood alcohol concentrations as low as 0.025%.
I worked in the electronics industry a while ago. Included was the development of screening breathalisers - the sort cops use on the side of the road.
As part of the research we discovered that alcohol is a banned drug in some sports - for instance rifle shooting and archery. It's not because it's dangerous, it's because in small quantities it improves fine muscle control and reduces tremors.
The research I saw also showed that the probability of a car accident decreased with small amounts of alcohol. That is with say a can of beer in you, you are a safer driver than a teetotal.
Driving safety improves up to .025 and then declines to 0.05 where it is the same as 0.00. After 0.05 safety drops rapidly, and 0.08 is getting pretty unsafe.
The only other downside of small quantities of alcohol is it's a depressant, so you are more likely to nod off later.