There's always the wrong-time-wrong-place situation, as well as running into an officer who is having a bad day or behind on his quota - lots of stories of police pulling over people who blow 0.0 on the breathalyzer and arrest them anyway for their quota.

Reminds me of the "I've got nothing to hide" mentality. Just because I'm not breaking the law, doesn't mean I want to make it easy for bad people to abuse me.