The mass balancing on the 737 47 57 67 on the cables was done to keep the stick force down for the pilot and to dampen the cables vibrational natural frequencies so the guides on the cables would not have excessive wear. As it is the cables still need tightening every 500 to 3000 hrs or so. Sorry about late date of this post, but just checked back in. Reason for manual backup is because no one system failure will result in a catastrophic crash of an airplane, thus all systems must have an independant backup which cannot have a common mode failure between systems.

Brian