I agree, even though the packs, were heavy, and uncomfortable to wear, especially the wood framed British pack, voluntarily leaving them behind was not a good option. Even if you had the good fortune to be able to get back to where the the packs were left, the chances of finding anything of value left in them was slim to none. In any case if the army you were fighting for won, you were probably going to driven on and not have a chance to get back and if you lost, you would probably not be too interested in stopping to find the pair of clean socks you had in the pack when there are lancers or cossacks breathing down your neck. In fact that would be when I would be looking to throw my pack away, along with anything else that interfered with the "Feet! Don't fail me now!" mindset I would be in.
Yet, having said that, the French, at least according to the British account, could not pursue the Russians after the Battle of the Alma becuase they had to go back for their packs. Of course the British had no such problems because, due to masterpiece of planning, the packs were loaded onto the ships first, underneath all of the other stores and equipment and the Army was landed with out their packs and made do with a folded blanket on their backs.