The problem is the power supply in the middle of that cable can fail causing the full voltage to go to the GPS and burn it up. There are a number of factors that can cause that power supply to fail - voltage spikes beyond design, too great of load (perhaps caused by water shorting the plug), mechanical vibration breakdown, etc, etc, etc. The higher the allowed voltage range of the GPS, there better the chances of it surviving in the environment we use them in. I designed and built power supplies to power my old eMaps on my bikes and ran into many of those issues. Even with spike capacity up to 45 volts I still lost power supplies.