The assumption behind the "if you have nothing to hide" claim is that the authorities will always be benign, will always reliably identify and interfere with genuinely bad people only, will never find themselves engaging in "mission creep" with more and more uses to put their new powers and capabilities to, will not redefine crimes, and even various behaviours or views now regarded as acceptable, to extend the range of things for which people can be placed under suspicion - and so considerably on.
It is all or some of naive, lazy and irresponsible not to be maximally vigilant regarding civil liberties and human rights, because it is a datum that the liberties of individuals are inconvenient for all states and their security services, and in dispensations where there are few if any restraints (think the Soviet Union, or even today's Russia - and China) it is liberty which quickly and comprehensively suffers.
Where an alert populace can use its liberties such as free speech to defend its other liberties vigorously, the universal tendency of states to increase their policing powers can be resisted: but even in such countries as the UK and US it takes real effort to mount and maintain such resistance. Consequently it is not acceptable to rest content with the "if you have nothing to hide" argument, for it is one of the most seductive self-betrayals of liberty one can imagine.
Update day 2020!
7 hours ago