First of all, privacy has an intrinsic value of its own. You don't need to have anything to hide to value privacy, it's simply a valuable thing in and of itself, and a requirement for true freedom. Not giving privacy its due value is an extremely naive position, and a dangerous one.
Secondly, not having anything to hide doesn't really matter when people (e.g., law enforcement) start sifting through that data, obtained without warrants, and decide that your behavior is "suspicious" or that you had the means, opportunity and motive to commit some criminal act that you had nothing to do with. You might think that you have nothing to fear if you've done nothing wrong, but tell that to Brandon Mayfield.
The more data someone is storing about all your movements, all your interests, everything you do, the greater the chance that these things will happen to you, and they will happen to more and more people in the future, especially as those who don't value privacy and freedom steer us toward a more proactive system of law enforcement. Yes, many of you will deride these as "tinfoil hat" comments, but dismissing the dangers of Big Data is the surest way to make sure that these things come to pass and that we erode and lose both our privacy and our freedom.