If I may say... I was not suggesting lines of dialogue should be labeled. Sorry if that was what you thought. It was only to indicate that second character for the point I was making. Labeling would be disruptive and a definite no no.
Dialogue should have a flow to it, and for this point, I am talking visually. In essence, each new paragraph should be a different speaker(stick to that--unless a speaker has a long speech and it needs to be broken up--even so, you can put in an action/reaction line from the other character to keep to that rhythm) and that should help the reader, and while overusing names is another no no in my opinion, maybe every once in a while it is okay especially if there are more than two people.
In a group, speakers would use names to get their attention, and that will also assist readers. I sometimes write dialogue with no addressed names for a scene, but that is usually well into a story. If you notice, there are no addressed names in the example given... it's not necessary, and usually not how people talk, though that does depend on the character and the emotion in the scene. An example there is "John! Are you kidding me?" In my previous example, names are used in the observations as a means of helping the reader and assisting the character development within the scene.
Hope this helps, and I really hope I didn't confuse things for you. Cheers! Gary