I love going to shows, and I love a drink, and yet I've never had any idea how normal people can drink at shows. Like, shows last for hours - I want to keep my energy up through the whole thing, which is not going to happen if I drink, and the last thing I want to do is need to go to some god-awful bathroom during a set. I'll gladly meet you at a bar to chat and have a drink, but if we're gonna rock or even catch some jazz, it's probably going to be sober for me.
(Edit: perhaps because shows used to be cheaper, people would just go see whoever and get trashed?)
People have different reactions to alcohol -- some might get very tired after just one drink, some get more energy with every drink they take until they suddenly crash. Also when you're at a show and you're dancing or jumping around and sweating, you find you need far fewer bathroom trips.
(Edit: perhaps because shows used to be cheaper, people would just go see whoever and get trashed?)