Most drivers that come from behind know that you're there. If not then something is wrong with your visibility (consider turning on lights during day time). The danger comes when that driver passes me. Thus I try to ride straight and not to close to the edge of the road. Because if I fall I will be under the vehicle that's behind or passing me. I don't see how a mirror would help me there. Also if I really need to know what's behind me I turn my head and get a better view than a mirror could ever give.

But then again that's just me. I wanted to state that a mirror isn't a must. Try it out and if it's not for you ride without one. You'll be fine.