Its sad when someone introduces you to a new community, telling you how much they love to drink. Then when you reply that you don't drink, said person tells you .. you know actually I don't drink either.
First up, very few of these parties are advertised as a place to taste alcohol. They are advertised as get wasted as much as possible cause its cool. And its pretty much every conference I go to.
But at any rate, I think the "original" purpose was the same as why people drink when they are back at home: To reduce anxiety and to make it easier for people to get to know each other. I think that is a worthy goal since to me the main purpose of conferences are to network, the talks come second.
But I know the same deal from sports tournaments where its also play hard on saturday, get drunk on saturday evening, play hard on sunday. I just sick around as long as there is someone not drunk to talk to, or the DJ doesn't suck so that I can dance. So it goes.
I also recently wrote on my Facebook page, that this is yet another form of excluding people. Especially if the only non alcoholic drink is water and pineapple juice. But yeah the difference is that nobody has seriously (well jokingly they have) implied that I am a worse programmer because I don't drink, where as it appears to happen still way too often that women are assumed to be booth babes or n00bs.
But getting smashed also has the tendency that people do stupid things and when a bunch of guys feel like they totally own a place in numbers while being totally drunk they might also start thinking they can "own" that female geek hacker. So yeah maybe there is where we get back to the sexism debate.