The premise of the "nothing to hide" argument is that only those engaging in criminal activity would object to being spied upon, and thus those who object must therefore be engaging in criminal activity. This is an attempt at intimidating people from disagreeing.
The reality of life today, is that everybody has opinions and statements which could be used against us by a hostile entity which is possession of our intimate communications. Look at the people who have had their careers or businesses damaged by it being revealed that they had opinions, or supported positions, that others disliked or found offensive?
Whether opposing gay marriage, or being gay and not wanting to be outed, whether being a conservative in a left-wing industry, or being a left-winger surrounded by conservatives, there are things we prefer to keep to ourselves.
This is especially true in a time when the powers-that-be have a habit of leaking private information about those who displease them.
Excellent way to follow the logic of the argument.