I install the "jumper" between the hot and cold lines BECAUSE the heater is metal if water heaters in a particular house seem to fail too frequently. It is my belief, valid or not, that when water heaters fail within a few years of installation, especially if it happens to more than one, that there are residual currents leaking into the hot water piping. As these currents, and they are not detectible by common means, gravitate to the cold water pipe and then to the earth, they eat away at the stell water heater shell. The jumper lets the currents bypass the heater. This may or may not increase the life of the water heater, but I had one house where one heater failed. I installed a new one, and in a matter of months it also failed. The top of the water heater looked like someone had taken a can opener to it from the hot pipe to the cold one. I installed the new heater AND a jumper, and it has lasted at least 8 years now.