Well the whole point of meal replacements & whey protein shakes is so that your muscles can almost immediately absorb the whey protein into the muscles right after a workout when protein, especially whey protein is essential. You need it right after a workout. In my opinion, drinking them during a workout is useless. If you've eaten a good solid meal consisting of 20-30grams of protein 30grams of complex carbs & had some citrus fruit an hour before training along with some water before your meal, than you'll be fine until after your workout.
To me, drinking them during a workout is just wasting money & the protein. No offense or criticism intended.