Many people are rightly focused on how the state as a whole could abuse the vast databases of information on regular Americans, but it should be noted that is not the only concern with such programs. Even if the “Government” acts in a mostly benign manner with this data, there is still the real danger of what rogue segments of a government agency or even individual contractors/bureaucrats could do.
In his interview with Glenn Greenwald, Edward Snowden acknowledged that his position allowed him, and many others, the ability to access huge amounts of information about individuals. Snowden said he had “the authorities to wiretap anyone from you, or your accountant, to a federal judge, to even the President.”
This is a huge amount of power to put in the hands of individuals. On an individual level it could be abused to pursue personal vendettas, stalk old relationships, blackmail people in power, advance political goals, or illegally access information to be used for financial gain. The number of nefarious purposes this level of power could be used by a rogue individual or small group is almost beyond count. The same secrecy that hides the existence of these programs could make it easier for individuals to hide their abuses of it.
Even the cynical argument that “if you did nothing illegal, you have nothing to worry about” does not address this issue. It is not a question of whether you can trust the “Government” as whole not to abuse this awesome secret power, but rather can you trust every single one of the thousands of government employees and private contractors who work on this program not to abuse it? Without even knowing about the program and what checks it may or may not contain, the answer is “No.”