If you have nothing to hide then you have nothing to worry about.
What is legal and normal now may be illegal and dangerous later and the then government could go back and look for "troublemakers" by examining old data to check for subversive comments. Good luck explaining your comments from 15 years ago when you get dragged off to a secret court for "terrorist" behaviour based on comments and opinions of many years ago.
What about supposedly innocent tracking data. Suppose you had to visit the dentist several times in quick succession and suppose your dentist office was located very near to a known terrorist cell. Now you are in a database of people to watch. Several years later you have to make repeat visits to some other location which by sheer coincidence is near another terrorist cell. You are now a hot hit and this triggers a series of events starting with the IRS doing a full audit on you and your friends /family potentially followed by much worse.
This is not science fiction but easily plausible. The more data you have, the easier it is to establish causal relationships which are not real, but could have real consequences when some government or other organization takes an interest in you.
In reality the systematic break down of privacy and systematic increase of power to those who are committing them eventually leads to oppression and near total control.
http://www.spiegel.de/international/germany/stasi-files-revi...
Unless you've got something to hide that you're worried about...?
Clearly, hiding anything is evil and people hiding anything are isomorphic with terrorists. :|