Do Anti-Discrimination Laws Actually Have a Reverse Effect?

We recently came across a passage about the law of unintended consequences while reading the book Super Freakonomics. One particular paragraph noted that while the Americans with Disabilities Act (ADA) was passed to safeguard disabled workers from discrimination, data shows that there has actually been a net decrease in jobs for disabled Americans. Specifically, the book claims "[a]fter the ADA became law, employers were so worried they wouldn't be able to discipline or fire bad workers who had a disability that they avoided hiring such workers in the first place."

Do you believe that anti-discrimination laws actually have unintended consequences like those outlined in Super Freakonomics? Have you actually encountered any such unintended consequences? Do you think anti-discrimination laws are still necessary?