Custom background images may seem like a good idea, but the copyright issue alone would scare Microsoft and Sony, as most people would be using images from the net to create them. The course file sizes would dramatically increase too, unless HB stored them all on their server. Even if HB supplied their own collection of 2D backgrounds there would still be lighting problems, especially if HB ever allow the player to choose the time of day for their rounds in the future. (something I hope will happen, but probably won't). Course loading times would also be longer, waiting for background images to download.
Backdrops would certainly help designers create more unique looking, or more realistic courses - I just don't think HB will see them as a viable addition.