DFW said somewhere (I can't recall where) that he wanted the reader to have the physical experience of moving back and forth, and that the process of moving and flipping sort of echoed the jumping between the years of the chapters and story. Or something :)

I found it annoying initially, but after I read that (when I was maybe 1/3 through), I did come to appreciate it a bit more. Maybe I'm just impressionable.