I've always hated that phrase, because it's objectively, provably bullshit. Almost everyone has some experience of a friend of acquaintance who become someone other than themselves when they got hammered.

It's which self you regard as the "real" self that matters here - alcohol tends to make it hard for people to maintain their habitual masking;.

Your otherwise calm friend who starts barfights when he gets his drink on? Probably not as calm as he seems on the surface.

I hate the phrase because it presumes that the person one becomes for less than 1% of their conscious life, while under the effects of a mood-altering drug, is a greater representative of their true self than their behaviour of the other 99%+ of their life while they are sober and clear of head.

The problem here is that there are a bunch of other situations in life for which being inebriated is a reasonable proxy. Sleep deprivation, extreme stress, social isolation, etc.

New parents often suffer from all of these! If you are a shithead when drunk, are you also going to be a shithead to your wife and newborn child when they most need you to be level-headed and calm?

Similarly for any sort of dangerous job. If I'm going to be sailing offshore for weeks alone with someone, you can be damn sure I'm testing ahead of time how they handle stress.

You've made an interesting point, but then, roughly in the ~theology of no-virtue-without-temptation we have this classic ~counterpoint ...

Batman Begins - "It's not who I am underneath, but what I do that defines me."

I've met/worked with plenty of people who seem very nice under normal conditions, and become complete assholes to coworkers/friends/their own wives and kids when they are under stress...

So I'd amend that to something like "what I do in stressful conditions defines me". Anyone can act cool as a cucumber before the shit hits the fan

(Batman's whole existence is stressful. He specifically gets a pass on this one)

Batman is a trustafarian. Why does he get a pass ?

I would say that people cannot be "other than themselves". You will just see their other side when they are drunk. And it is quite often the darker, more primitive, violent side, which they suppress under normal conditions, knowing well that other people won't like it.

There is a sentence in some Stephen King's novel (and King is a recovered alcoholic to boot) about the main hero. IIRC it sounds like this:

"There was a dangerous dog in his mind. Sober, he could keep that dog on a leash. Drunk, the leash disappeared."

Maybe you are observing the leash disappearing.