Alcohol has little to no nutritional value. It is also high in calories at 7 calories per gram. As these calories are empty, they get converted into a fatty acid, which has a high chance of being stored as body fat.
Food is fuel, so whatever you are putting in your body, you have to weigh the calories with what you are getting out of it. For example, a sweet potato can be 100-150 calories, depending on the size. However I get tons of vitamin A, which is an important antioxidant, helps with eye, skin, bone, and immune system health. I also get a good amount of fiber and potassium with is an important electrolyte. Potassium may help counter the effects of excessive, artery constricting, sodium intake. So you see, eating the sweet potato is worth the calories.
Alcohol is a diuretic. It causes the body to dehydrate, which is bad for someone with fitness goals. The circulatory system is the body's transportation, and it needs water. When you are dehydrated, imagine your blood moving like syrup. If you are hydrated, it moves like, well, it moves like water, which is much faster than syrup. Dehydration means headaches, and slower muscular recovery. Who wants to go to the gym feeling like that?
The body perceives alcohol as a toxin, and because of that, the body works very hard to eliminate it. It puts everything else on hold to get rid of the alcohol. The reason alcohol puts such stress on the liver is because most of the alcohol elimination will occur in the liver.
Along with nutrient breakdown and storage, detox is the most important job of the liver. The liver decides what foods get disposed of, and which ones get broken down into glucose to be used by the body. The liver also breaks down nutrients to disperse throughout the body. Iron, B12, and glucose are also stored in the liver.
So continuing on, the enzyme necessary for alcohol elimination, "alcohol-dehydrogenase", is expensive as far as nutrients are concerned. The most valuable nutrient used is zinc, with is used in over 300 bodily reactions that we know of. So if you use up all your zinc getting rid of alcohol, there are a bunch of other bodily functions that are now trying to find work-arounds. The body is a trooper, but this is why excessive long term alcohol use can have lasting harmful effects.
In closing, it is understood that most people will have the occasional drink or two, but drinking heavily every weekend, or every day will, internally, cause everything else to take a back seat to repairing all the damage done by the toxin you chose to drink.