Just for the record, I too keep my boxes.
However, it's funny - this is really an issue of our own making. I believe very few of us want to keep the boxes, but we all know that it helps resale value. Why does it help resale value? Because the person buying the used gear wants the boxes. Why do they want the boxes? Because it helps resale value when they go to sell it.
Bottom line is that I doubt most of use care one way or another about boxes, but the private used camera market has created a "gotta have the box" loop that we're all stuck in.