Historically, milk has been considered to be a near-perfect food for humans, given its high calcium and protein content, and the fact that raw milk is tremendously easy for a healthy person to digest. Hippocrates advocated a raw milk diet to cure tuberculosis, and both the Masai and Swiss based their diets on milk. It's impossible to purchase raw milk easily in the US, and pasteurized and homogenized milk borders on indigestible for most people. Whereas raw milk merely ferments in your intestines (which actually makes it better for you), pasteurized milk putrefies. Additionally, pasteurized milk is shown to be among the top three food allergies, and has symptoms ranging from ear infection and bad breath to asthma, abdominal cramping, diarrhea, croup, and asthma, even in people who are lactose tolerant.
Even skim milk contains 205g of sugar per gallon and has a higher Glycemic Index than fatty milks. Thus, in straining the heck out of your digestive system, you're getting a paltry 145g of protein for your efforts, not all of which will be digested due to the fact that the fat molecules in homogenized milk are broken down into smaller parts and become a stealth delivery system for the proteins that put them directly into your bloodstream, causing allergies. Pasteurization takes care of the rest, destroying all of the enzymes that would aid in the digestion of this chemical monstrosity.
If you want protein, eat meat.
If you want calcium, eat dark leafy greens.
If you want to get them from milk, make friendly with a farmer and get it raw.