What if the government leaks the information to the public? What if the government mistakenly determines that based on your pattern of activities, you're likely to engage in a criminal act? What if it denies you the right to fly? What if the government thinks your financial transactions look odd—even if you've done nothing wrong—and freezes your accounts? What if the government doesn't protect your information with adequate security, and an identity thief obtains it and uses it to defraud you?
Even if you have nothing to hide, the government can cause you a lot of harm.
By Daniel J. Solove
Little Brother, by Cory Doctorow.
Still not convinced? Better check this out!