Quality versus quantity. This is about wholesale surveillance -- gathering as much information as possible on as many people as possible. That does not mean the information is better or even useful.
People easily imagine the worst case scenarios, but the okay case scenario is surely more typical, e.g. when a man travels to eastern Pakistan to allegedly visit relatives for several months, the FBI can look at the 9 people he spends time with rather than creating dossiers on the 112 people who so happen to attend the same mosque.
(Whether and how the Constitution cares about the "good case", "okay case", "worst case" is a different question.)
Maybe the FBI/NSA is so good that they never see false positives. I doubt it, though, given how many non-terrorists have been harassed at airports, had their peaceful protest groups infiltrated by undercover agents, etc.