* Industrial espionage -- it's big business, and I'm sure it pays better than being an NSA analyst.
* Foreign espionage -- since this gives unlimited querying power to every agent, a single "turned" agent could inflict massive damage on U.S. government and industry interests on behalf of a foreign power. The potential for double agents is huge.
* False positives and guilt by association -- being flagged as a "person of interest" and then essentially persecuted because you have fringe ideological interests, are looking up a lot of info on terrorism for a book project, have a friend who knows radical Muslims, etc.
* Corrupt use in political campaigns by incumbent politicians with access -- obvious.
* Blackmail and other corruption.
* Use by government agencies with access to spy on other agencies.
... I'm sure creative people can think of more.