Obviously, there's a lot of discussion right now about what information should be considered private, and whether or not people really have nothing to hide. I believe that people have a lot of information that they want to keep private: even when they say they don't.
When I meet people that say they have nothing to hide, I 'play' one (or more) of three cards.
The first thing I will do is ask them how often they masturbate. I know it's a fairly taboo question, but that's the point. It's not illegal, it doesn't hurt anyone, but not many people want to answer the question and I will push them a bit before I change tack. A lot of people think that this is a stupid question because it doesn't make a difference and that it's meaningless. Apparently that makes it somehow mean that they still have nothing to hide, but anyway.
Sometimes I choose to play the "we don't know the future" aspect. One of my cousins is gay. That is not illegal where we live, but there are places where it is and let's face it, there are no doubt people even in our community that would prefer for it to be illegal. In the future, what if it became illegal? All of a sudden, all of that knowledge of what I didn't need to hide becomes a problem.
Let me reiterate that: right now, my cousin has nothing to hide, and I have nothing to hide in knowing that they're gay. In the future, if it became illegal, there's a ton of information that suddenly we may wish to have been kept secret. Unfortunately, that stuff that we didn't have any reason to hide suddenly becomes something we may wish we were able to.
Finally, depending on who I'm talking to, sometimes I will play the family card. If the person I'm talking to has a child (especially a daughter) I like to ask them how they would feel if in the future that child was dating a good government employee that had access to all sorts of information. Then I begin to wonder, what would happen if they went through a messy break-up? I wonder, what if that ex turns out to be a little jealous and maybe a bit of a stalker? Would you support, then, there being a heap of information on your child, being easily accessible to this ex?
I guess a lot of my arguments come down to this: you don't know the future. In this, you have no idea what you do at the moment that may be considered dodgy behaviour in the future.
Even if you think that that's not a problem, there's a very good chance that right now, you do stuff that you don't want people to know and as far as I'm concerned, the very fact that you haven't considered that means that you haven't spent nearly enough time thinking about it. And to be honest, that frightens me just as much.