Apparently light drinking is not enough to protect your heart, at least according to this editorial in the Lancet (no free text). There are no randomized controlled trials of alcohol consumption so only observational studies exist, and these do show evidence of a cardiovascular protective effect even with light consumption (1-3 drinks per week) versus non-drinkers.
Of course, these kind of observational studies only show correlation not causation, and inferring causation can lead one astray (I've posted a lot more about this here). For example, epidemiologic studies of hormone replacement therapy showed a protective effect against heart disease and breast cancer, whereas later, randomized ones showed HRT actually increased the risk of these diseases. Why? The most likely culprit seems to be the "healthy user effect." Women who worked out, ate well, etc. were also likely to take HRT. These so called "confounding" factors more than made up for the deleterious effects of HRT.
Could a similar situaton exist for alcohol consumption and heart disease. Maybe. One study of light to moderate vs. non-drinkers showed that for the majority of risk factors, non-drinkers were at less risk.
However, the arguments for a protective effect of heavy alcohol consumption are more compelling. Heavy drinkers are in general much less healthy than light or non-drinkers as most alcoholics smoke, don't exercise regularly or keep their high-blood pressure well controlled, etc. Despite the fact that these confounders work against them, they still are protected against coronary artery disease:
Although less palatable, there is more compelling evidence for a coronary-protective effect of moderate to heavy drinking than for light to moderate drinking. In heavy drinkers, confounding will obscure rather than exaggerate any coronary protection because of their heart-unhealthy behaviours. The observations of relatively “clean” coronary arteries in autopsy studies of alcoholics are also consistent with a coronary-protective effect of heavy drinking.So bottoms up!