I have become so accustom to my helmet mounted mirror that if I ride without it, I feel naked. I too am flexible enough to turn around and look over my shoulder, but why distract yourself from the road for such a long time when a quick glance in the mirror will do. I want to see that a car approaching from behind has his left side wheels over the center line. This tells me I'm safe in that I know the driver actually sees me. If the car is holding his line in my lane I know I need to stay as far right as possible or even get ready to bail off the shoulder. If I had to turn around for this every time a car approached, I would never see the road ahead.