Well, I don't have a problem with people who want to voluntarily give up this information. However, I do have a problem with (by virtue of federal law) people forcing me to give it up against my will.
I have nothing to hide either, *today*. Having something to hide doesn't necessarily mean you've done something wrong. Having something to hide can come as a product of someone else deciding you've done something they don't like, which history has shown time and time again that it can be something as simple as race, religion, or even philosophy.
I'm heavily involved in politics on a local, state, and national level. I often have a different viewpoint than those in my own party and I am suffering because of it. People are vicious and when you work in an arena where you're challenging longstanding ideas within your own group, people come out of the woodwork to attack you. I worry about what could happen as a result of stirring the pot, but someone HAS to do it or we are in some deep trouble. The last thing I need is someone who has it "out" for me, to have access to my children's blood and DNA.
Not everyone in this world is a "good" person. In fact, as we can see by the cases handled here, there are a LOT of very bad people out there. We have to protect ourselves proactively. We don't want to be looking back in 20 years thinking "oh, God if we only could have stopped that bill from passing". Err on the side of caution.