Atkins does allow alcohol once you're out of Induction, but obviously in moderation.
Firstly, a lot of alcohol has significant amounts of carbohydrate (beer, cider, wine for example) and those carbs have to be counted into your daily allowance. Whilst that's okay for an occasional drink or two, you don't want to be replacing your
good carbs (eg. vegetables) for empty carbs on a frequent basis.
Secondly, even if your tipple of choice is carb-free (gin, vodka, whisky....spirits, in other words), they still contain calories, and even on a low-carb diet, if you take in too many calories you can fail to lose weight. In addition, you have to consider what you're mixing those spirits with. Water is obviously fine, but full-sugar mixers clearly aren't, and diet mixers (eg. slimline tonic) count towards your daily allowance of artificial sweeteners so can't be consumed heavily.
Thirdly, alcohol is a sugar derivative and as such is a fuel of preference for the body. As soon as you consume it the body stops burning fat and burns the alcohol instead, hence people can find their weight loss slowed or stalled. This is clearly not a good thing!
You also have to consider that under the influence of alcohol your judgement is likely to be impaired and you are therefore MUCH more likely to eat things that you wouldn't have done otherwise eg. the typical carby nibbles that go along with it.
For myself, I find that the effects mean I not only have to fight the urge to snack on bad stuff at the time, but even the next day I find I'm drawn to it, and that puts me in a really cranky mood
.
So it's really a personal call. If you don't mind risking your weight loss short-term and you think you can handle alcohol without binging on it or eating inappropriate food and can get straight back on the diet the next day even if you do go badly off track, have a drink or two. I've had 3 separate occasions where I've had alcohol since Xmas - 2 G&T's each time - and I've gotten away with it (just!), but we're all different.