How often do you need your seatbelt? Should you still wear it anyway? And there are situations when wearing a seatbelt can kill you where you would survive without it.
The idea is the same with cops. A properly trained cop carying a weapon all the time would be safer overall by being able to react to violent situations while not posing a threat in non-violent ones. The issue is how incredibly undertrained most US cops are.
And yes, if they are not sufficiently trained, they should not carry a gun period. Not in their car, not anywhere.