As bike commuter and recreational tourist for about 40 years, I rode for many years without the aid of a mirror and was frequently cranking my head around to see what was behind me. That action made my bike wobble, which can be dangerous when riding in traffic or in a group. A friend finally convinced me to try a helmet mounted mirror and I haven't looked back since. Well, yes I have looked back, but I don't need to jerk my head and neck around to do it!
Once you become accustomed to using a bike helmet or eyeglasses mounted mirror using monocular vision you find yourself referencing it as frequently as you use your car mirror. A glance every 15 seconds or so, depending on the traffic conditions, gives you information about the trajectory of each vehicle approaching from behind. It provides enough warning to adjust your riding behavior if necessary. You may decide to "take the lane" when there isn't room for a vehicle to pass safely. My experience is that the actions of the lead driver in a line of traffic is likely to be followed by the drivers behind. If the lead driver passes unsafely, you can bet the others will also. Using a mirror may provide enough warning to decide your life is at risk and simply get off the road until the danger has passed. Simply put, using a mirror can safe your life.