It's about common sense and whether or not you want your life to depend on random people not being negligent, respecting rights, being able to see you, not losing control of their vehicle, etc.
I'm sure you all see the news and what crazy/irresponsible people there are out there. Well, those people drive cars too.
Also, what if your riding along in the bike lane and you have some sort of accident that causes you to swerve or fall into oncoming traffic, perhaps in the way of some conscientious, respectful driver? Regardless of responsibility, you get hurt.