Well, the only droids that had backpacks in the movies were the plain infantry Battle Droids. With Prophecy's droid model, the blue and red droids had a very large colored area on their back, so I think that it wouldn't be much of a problem. If you see a droid with a backpack you'd know right away that it's an infantry droid. Prophecy set up the command droid to have a backpack as well, but the command droids didn't actually have backpacks, just antennas. The backpack surface can always just be turned off.