I've never had a problem with moderate alcohol consumption during my games. My games have always been occasions for myself and my fellow gamers to 'unwind', and if some people want a pint of lager or a glass of wine during a session, I see no reason not to indulge them.
If your players are mature (i.e. not likely to become drunk and disruptive), what's the harm?
(I know that I would not be thrilled to game with someone who refused to let me, a responsible adult, have a beer during a game. That kind of patronizing attitude would really rub me the wrong way.)