Why would it shorten life of rectifier? If you have shunt-type rectifier*, load on it will be exactly same whether it's charging battery or not. And bad battery will actually draw less current from rectifier than good battery.
* Shunt-type rectifier always runs at 100% full-load. Regardless of how much bike uses (lights, ECU, battery-charging), flow through rectifier will always be 100% of stator output. The split between consumption may vary:
5+3+2+15 to ground = 25amps (2-amps charging good battery)
5+3+0+17 to ground = 25amps (0-amps charging bad battery)
Well, I'm nearly 80 so some things have changed since I was 18. This is part of it tho:
There are different reasons why the regulator rectifier fails. One of the top causes is heat. Some motorcycles are known to have the part located near the radiator or other locations that are either close to heat producers or restrict air flow. Depending on the location of the regulator rectifier, the part can easily overheat. Other causes for a dead regulator rectifier center on the battery. Ground connections are important for good voltage, and if there is faulty voltage, the regulator rectifier can run hot. Bad grounding, corroded battery connection and poor or loose battery connections will cause faulty voltage.