I bet it is very useful. If you have one or two suspicious persons who happen to be a member of a particular lodge, church, mosque, etc. you can probably usefully winnow down the list of associates to further investigate without a large and intrusive effort investigating the entire group.

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.)