I just scanned this from "Eat, Drink, and Be Healthy" by Walter C. Willett, MD. Co-Developed with the Harvard School of Public Health.
Interesting to note that "the dose makes the poison" is clear here. A small amount will help, a lot will kill, and failing to drink at all appears to be about as bad as slightly over-drinking. Alcoholics Anonymous and most medical doctors will admit that moderate drinking is healthy, but recommend against it because of it's easy potential for abuse.
This is also correlational data, like most health related studies, this one looked at who lived the longest and drew correlations to their diet. It could be that easy going relaxed and long lived people simply have a habit of drinking moderately.
However, it is likely that alcohol helps in moderation because we know that blood thinning drugs, including aspirin and chemicals in some plants, can help to prevent heart disease. It also appears not to matter if the alcohol is wine, beer, or hard liquor.