Your snobby cocktail friend may fly into a tizzy (or worse, a lecture) at the utterance of “shaken, not stirred” but, though their feelings on shaken martinis are valid, shaking a cocktail can be good, actually.
Deciding whether you should shake or stir a cocktail has nothing to do with “bruising” the booze — booze doesn’t have a circulatory system — and everything to do with your ingredients and their relative densities.
In the words of my favourite bartender, Daniel Casto, “Do you want something stirred and boozy or shaken and refreshing?”
If your drink is all alcohol — as in a martini—the densities are all going to be, if not the exact same, extremely close, meaning you won’t need a whole lot of agitation to bring everyone together. However, once you start adding juices and thick, viscous syrups, you’re going to need to create a little more turbulence to make sure you have a homogenous beverage without a pool of cold syrup at the bottom.
Also, there is a matter of mouthfeel. Both shaking and stirring chill and dilute a beverage, but the violence of shaking also works to introduce air (tiiiiny bubbles) into your drink, which can be quite pleasant if appropriate. I can only speak for my self, but I do not want a frothy martini. I want my martini to satiny smooth and extremely cold, and diluted just enough to temper the harsh bite of the ethanol.
But if I’m drinking a daiquiri? You better shake that thing — I mean really slam it around as hard as you can for at least 10 seconds. The extra physical force will make sure that everyone gets to know each other real well, no matter their viscosity, and create a light and airy cocktail which, as it turns out, can be a very good thing.