I will do my best to help you understand it since you asked. A bullet can go through a car door, a car windshield, a car window, a bumper and various other things the cop is surrounded by in his patrol car. If, like I stated before, the assailant pulls a gun, the officer sitting in the car has nothing but a windshield between him and a bullet to the chest or head, which just so happen to be the main targets of opportunity when shooting a weapon at someone. Yes, he can duck like you said, but all the assailant has to do now is walk over to the car and shoot through one of the windows, while the officer is still stuck in his vehicle having a difficult time removing his weapon from his holster because it is hard to pull it out while sitting down with a seat belt on. Especially if you are ducking to your right which is causing you to lay on your weapon, unless you are left handed of course. Therefore, if the cop steps out to the side of his car and orders the person to do something, he can then retreat to the rear of the vehicle if necessary where there is a better chance of him getting cover from the possibility of gun fire. This also depends on how far away the cop is from the bike. If he leaves a lot of room, I mean like 8 car lengths, then maybe I can see the officer choosing to use the mic to direct the vehicle operator what to do from the patrol car.