I definitely appreciate having a mirror, but don't consider it absolutely essential. I have a brifter-mount that I used for about a year and loved it, and got one for my wife. Discovered that our storage arrangements in our small apartment are too tight to store both bikes with mirrors attached. So now mine mostly sits in a drawer, I take it out to go touring or on longer rides, but not for the daily riding around the city core. I found that once I re-adapted to riding without a mirror, most of its functions are replaced by my ears, so while the mirror is very useful for being aware of overtaking bikes, I'm just as aware of motor traffic without it.