Likewise, every time I ride I see motorists who are not following the rules. The most common infractions by the cars are failing to signal for turns, failure to yield right-of-way to bikes and speeding. The failure to signal is especially irritating because on a bike we need to know what the cars are going to do!
I always ride with a mirror. It allows me to know what is going on all around, not just in front and on the sides. With this awareness, I can have more control of the situation, which in some cases means taking the lane. Without a mirror, I would feel completely at the mercy of motorists, many of whom are drunk, texting, tired, etc. No way am I venturing out on the roads without the protection a mirror provides.