From my perspective, there seems to be an inverse relationship between seniority and shelf use. The higher the seniority, the less likely the shelves will be used (the old "this is how we've always done it and I'll never change" mentality). I personally love the upper shelves. Put all the carry-ons, duffles and hanging bags there so you can better stack and arrange the larger suitcases on the floor. Also reduces bending over to place or pick up small items on the floor. Win-win.
I'm struggling to find a use for the bottom shelves, though. They would have been nice to keep bags off the wet floor in leaky Heritage cars. In the Viewliners, they just take up space. Using them greatly reduces both vertical and horizontal storage space.