By seeing the side of your car.. you will not see a lot of other objects which are right next to you. I have a great picture which I took in Manhattan traffic and you will be surprised if you just do a test yourself.
you can always move your mirrors while parking or certain conditions but for highway driving the guide is beneficial. It looks little difficult to accept that you don't need to see your trunk but this is what auto safety expert recommend. I was almost killed in my crash as linked in opening post. It was my fault that I also didn't see a truck which was right next to me. Yes I used to see my trunk but not now and it's so effective.
