A healthy diet requires protein, fats, fibre, vitamins, minerals carbohydrates and water. The reason athletes use protein is because by exercising they damage their muscles, and protein needs to be available for a fast recovery.
If you consume just the right amount of (whey) protein - you'll be fine, it is usually high in quality and has all the amino acids necessary (that protein can provide).
But here we run into our first problem, you still need to consume enough fats, fibre, vitamins and minerals. So chances are, if you're not careful, you will overshoot your daily requirement of protein by eating other food (which also has some protein). Too much protein not only can overload your kidneys (which are desperately trying to get rid of the excess) but will also be stored in your body as fat. This is especially true because protein powders are very concentrated, and you don't need much of it when you're not working out.
So to answer your question, it's not a bad idea if you meticulously keep track of exactly what and how much you eat per day to meet your nutrient requirements and not overshoot them. You also need to have knowledge of your body type, rate of metabolism and other factors. Its just much more inconvenient to do this when you're not working out, so I wouldn't recommend it personally.
If you were thinking a protein shake can replace a meal, then absolutely not. You could make some super-shakes containing a large fraction of the nutrients you need with some crafty planning, but you won't escape eating something along with it anyway. Do yourself a favor a find some time for a nice lunch. If you're really low on time, prep the food at home and carry it with you, but when it comes to food - be nice to yourself.
Hope this helps!
