At first, I figured none of the studies suggested causation, only correlation. At least one study suggests that the apparently cognitive benefits in older adults who drink moderately (as opposed to not at all) are nonexistent after controlling for intelligence in young age: http://www.ncbi.nlm.nih.gov/pubmed/12878923.
But then I came across one damning section: "Alcohol Abstainers Who Begin Drinking Reduce Their Risk of Cardiovascular Disease":
During a ten year study of 7,697 non-drinkers, investigators found that 6% began consuming alcohol in moderation. After four years of follow-up, new moderate drinkers had a 38% lower chance of developing cardiovascular disease than did those who continued abstaining. Even after adjusting for physical activity, Body Mass Index (BMI), demographic and cardiac risk factors, this difference persisted.This study is important because it provides additional strong evidence that the reduced risk of cardiovascular disease among moderate drinkers is a result of the alcohol itself rather than any differences in lifestyle, genetics, or other factors.
(from King, Dana E., Mainous, III, Arch G. and Geesey, Mark E. Adopting moderate alcohol consumption in middle-age: Subsequent cardiovascular events. American Journal of Medicine, 2008 (March), 121(3).)
So I may have to reconsider...