We're talking about the US, right? And what, specifically, about J.E. Hoover? We're probably not going to argue that he was a bad guy. Whose presidential nomination or campaign did J.E. Hoover derail? And again, let's stipulate that this was a terrible, autocratic person who abused his decades-long position in office.
Not a presidential campaign, but Hoover was really not keen on MLK and spent a lot of effort attacking the civil rights campaign: http://time.com/3582004/fbi-letter-hoover-mlk/
Outside the US, the examples of these abuse are numerous. Probably the most spectacular was Francois Mitterrand in France, who was wire tapping his political opponents and some other "persons of interest" (famous actress). And by using anti-terrorist powers by the way!
And having this power is not only open for abuse from the President. Lots of people in the chain of command will probably abuse it. It will also be open for abuse by law. You don't know what is OK today but will be inappropriate or illegal in 20 years.
All of this has nothing to do with the US but with human nature. The reality of any large society is that you have a portion of the population who are crooks, bullies, rapists, egomaniacs, corrupt, etc. Designing a system that relies on everyone now and forever to be honest is bound to fail. This is why the US constitution introduced so many checks and balances. And one fundamental check and balance is to limit the power of the state. And the state having so much information on its citizens, on everything anyone says to anyone, on where anyone is at any given time, on who talks to who, on who reads what is way way too much power.