Wired.com has an interesting article that references this one. They claim that perhaps the positive effects of alcohol on lifespan are related to its role as a social lubricant. In other words, it may be tied into making us more social and therefore happier or less likely to suffer from loneliness or depression.
Regarding alcohol, something else that should be considered, is the fact that in addition to its long term effects there is also some acute risk of death. An individual's likelihood of drinking to the point of alcohol poisoning or to be in an alcohol related accident should be factored in.