The layering principle is more important the longer you are exposing yourself to the elements. If you are going for a day hike, or even a 3 day trip, in moderate weather the layers seem unnecessary. On longer treks, you are going to be subject to various temperature changes of which you want to be able to control your body's temperature. While you are dealing with crossing stream, hiking in wet socks, cooking over a soda can, fighting off bugs, setting up your shelter, dealing with hot spots/blisters, layering will ensure that you are neither shivering or wiping the sweat off your brow, granted it takes practice and knowing your body.