I ride in urban traffic without a mirror. There are always cars passing me. If I looked in the mirror, I'd pretty much be guaranteed to see a car behind. If I looked in the mirror and saw a car head straight up my backside, I don't guess there would be much I could do about it. Mirrors are a convenience to some, and others find them a distraction.
In past post, I have described how over many years, the use of a mirror has allowed me to bail off the road early enough to avoid being hit from behind. In one case, I still got hit, but it was with a folding side mirror rather than the bumper up the ass that would have otherwise occurred.
Ride enough miles, and eventually a mirror will help keep you out of the hospital.