The phrase "I have nothing to hide" is inherently flawed. It implies that the speaker is a completely complacent citizen, incapable of opinion or any sort of activism that would be met with government resistance. It also implies that the speaker has never had confidential information shared with them, such as a friend asking for advice in a time of trouble. And that the speaker has never had to, for example, hide their job hunting from their current employer. And because these are all perfectly valid and common things that are worth hiding, it follows that everyone has something to hide, no matter how trivial.
Not everyone, however, is so benign. During the civil rights movement, anti-segregation activists were frequently placed on government watch lists so that they could be surveilled by the FBI. Their advocacy, which were at the time in conflict with the state, is now regarded as noble. There is nothing preventing a similar situation from taking place today, especially under a Trump administration.
Fortunately, this took place during a time during which the infrastructure simply did not exist for everyone in the United States to be monitored. Today, however, this infrastructure does exist, and it is being actively used to target and monitor activists whose work is in conflict with the government. And while such cases take place in the United States, as revealed by the Snowden leaks, they are even more common in China. Surveillance being used as a tool to depress dissent is not a matter of how a particular surveillance apparatus is used. Stifling dissent is in the nature of surveillance itself.
As a result, it is imperative that such systems are quickly taken out of operation. The mere knowledge that one is being watched has been shown to prevent activity that is perceived to be in conflict with the prevailing political and social ideology, and so no matter how ethical or noble the government, a surveillance apparatus goes against the best interest of a democracy.
Therefore, privacy is required if one is to make change. The ability for an activist to conceal their actions from the entity that they are working to change is required if they are to be effective. If the entity is aware of everything the activist does, they are in a much better position to smear the activist, fight back, and prevail.
Today, there are a plethora of technologies that allow for heightened privacy, but they are often underused. Because they often come at the price of convenience (for example, PGP, an established means of encrypting email, is very cumbersome to use), they are slow to catch on in a populous that is relatively indifferent towards privacy. As a result, those who do utilize the technologies stand out to the government and have more resources devoted to tracking them.
The solution is to normalize privacy. If everyone utilizes anti-surveillance technologies, those who truly need them will not stand out. "I have nothing to hide" hurts those who do. Hide, so that hiding doesn't stand out.