On a tasty episode of The Cocktail Spirit on the Small Screen Network, expert mixologist Robert Hess (previously) explains the importance of using the proper measure when crafting cocktails. Hess also notes that while there may be times when a bartender can free-pour ingredients, a measuring tool is necessary when creating a cocktail with multiple and distinctive ingredients.
There are two distinct camps that bartenders often segment themselves into, those that free-pour and those that measure. Personally, I am a strong proponent of measuring. I feel that the only mildly valid argument against it, is that measuring takes a little longer, and so in a very busy bar it might slow things down. …For some drinks, the proper measure is more important than others. One-Quarter of an ounce is not a very big measure, and it can be easy to accidentally over or under pour by that much when mixing drinks. Drinks such as the Old Fashioned, Manhattan, and Martini are such that being off a little bit may not be very noticeable, but when mixing drinks with tart citrus, or intense ingredients like Chartreuse, that 1/4 ounce can make a big difference.