The real problems arise if data is put together and interpreted against your will and probably without your knowledge. Imagine an ad network that combines your location profile, your Payback data and name/address and sells that to an insurance company which decides to not have you as a customer, because your are probably overweight (drives by car, no visits to any places associated with sport, buys a lot of food). Or imagine an automated alarm that is triggered when you try to enter the country, because you travel with three coworkers from Iraq and you have written an email to your friend telling him you intend to "devastate the USA" (meaning to beat the American branch of your company in a friendly soccer game after work). You cannot rectify these misunderstandings as nobody is telling you why you now always get the special attention of the TSA or why your insurance application is denied.
This is the stuff I really dread in the current focus on big data.
A great article on the same topic: https://chronicle.com/article/Why-Privacy-Matters-Even-if/12...
Are you so sure about this?
http://www.huffingtonpost.com/2013/11/26/nsa-porn-muslims_n_...
People send postcards all the time precisely because for the most part they don't care about privacy in their mail. In my personal life I can't think of a single example where I sent mail that I wanted to keep private (outside of maybe mail that had my SS#). I think most people are rather indifferent.
"If you hide your mail inside envelopes, does that mean you must be a subversive or a drug dealer, or maybe a paranoid nut" The main function of the envelope is to tied a bunch of papers together so they don't get separated in the mail.