Meal replacement shakes are good for you because they help you replace high-calorie or unhealthy meals with a lighter, more balanced option. They make the nutrition part of losing weight simple. Since meal replacement shakes are meant to replace a meal they contain essential nutrients, carbs, fats, and protein equivalent to a balanced meal. Meal replacement shakes designed for weight loss should also contain hunger blockers to keep cravings and appetite under control.
However, a meal replacement shake is NOT a protein shake. A meal replacement shake can and should have an adequate amount of protein, but a meal replacement shake is much more than that.