Isn't this just an extension of the 18 or 19% gray card idea?
Learning to see "light" with your eyes and setting your camera on spot-meter and getting several readings from different parts of a scene and figuring an average, and also knowing from experience that your dynamic range of your sensor is a little on the small side, and which ISO will give you the best results, and being able to figure which f-stop you want to use because the "sweet" spot of your lens is about f5.6, and how to read a histogram on your camera's LCD and which focus point, and lens diffraction and changing lighting conditions and white balance and . . . .
. . . . wait, what's that phone number on my hand? Is that the number of that hot chick I met at the bar last night, and didn't I tell her I was some famous photographer for some cool magazine or something, oh crap I had a lot to drink last night, and what is the exposure of my hand, is it my palm or the back of my hand, because I know exposure like the back of my hand, and I've got my hand in my pocket and do I need pocket wizards or should I just . . .