Alcohol should generally be avoided when doing any diet because it can hinder your progress. For one thing, alcohol is loaded with empty calories. Even supposedly diet-friendly drinks like martinis have plenty of calories, simply because alcohol itself is a huge source of calories, let alone the mixers drinks come with.
Not to mention that alcohol is notorious for opening one's appetite, so after that drink you might be thinking to yourself, "Oh maybe just a little munch won't hurt." Next thing you know you're elbow deep in a bag of potato chips.
Okay maybe it's not that serious for everyone, but that's my personal experience.