The other issue with shakes, if it's whey protein, is how fast they are digested. If you slam a shake with 50g protein you won't be able to make use of all 50g because of how fast it will digest. When you eat real food, the breakdown is slower and you get a trickle of aminos from the protein over a long period of time. Casein may solve this problem to some degree since it breaks down slower, but your still left with all the problems from drinking that many shakes in a day like others above have mentioned.