I notice that a lot, if not most cyclists don't use rear view mirrors.
I gotta tell you, cycling on the road with lunies on their phones and generally not paying attention does scare me.
I see mirrors are available... however. I they a useless bit of gear or just no one wants to be seen looking like a dork. Since I look like a dork anyway, I feel a mirror wouldn't be detrimental but could help me increase my SA.