I'm a regular cod4 player and it's always a good night in the weekends when I'm drinking. For the most part I'm top 3 of the tables, hax excluded. When you've played fps rts rpg as much as I have it makes little difference initially. But you can enjoy the social part of the game more. Ie, chat with other players while you're dead, or give key info to your living teammates especially in cod4. Some of the funniest chats I've had is with other players that are drinking. We compare notes, as to who's drinking what. And I can tell they are drinking cause their scores are usually less than what they normally are, so in that sense, yes there is a penalty.
At the end of the day I guess it comes down to why you play in the first place. For a lot of us, it's just to relax, and scores don't validate our lives. Some of the best times I've had is finding the hard to reach places in a map, or getting all the aircraft in battlefield 2 at one airport for example. Nothing like 3 heli's stacked ontop of each other and I'm sure I still have the screen shot somewhere.
Bottom line, it's online play and it's not serious(like playing for cash), so you just get to that point where you have fun with the engine. To me that's what it's all about. Recreation time.