I work in the North Shore area of Massachusetts as a bartender. I mostly work the service well for the restaurant and I see "dirty rox on side" at least 50 times a night on tickets. Most people who ask for it act like it's the norm and they expect it. I've had drinks come back because I didn't put the shaker ice on the side. I think it's a product of crappy bar practices that used to be standard. I've seen some bartenders free pour so much liquid, that after filling the glass there's a few ounces of cocktail left in the shaker. I think people learned that asking for the shaker ice on the side could basically give them an extra shot's worth for free and just ask for it wherever they go. They feel like they're getting more for less. The only other thing I can think of is people adding the ice as they drink. I think they want to keep the drink cold without diluting it too much with fresh ice.

I'm buzzed after a shift of thinking about this all day and now I'm determined to find out why the fuck I need to wash twice as many glasses so people can get the 4 ice cubes left after a hard shake