Well, they certainly can be a source of empirical evidence.
Not by themselves.
Individual witnesses may be reliable or unreliable. You seem to think that there is no way to tell whether witnesses are reliable. It has nothing to do with having “faith” in a witness.
Of course there is a way to tell if a witness is accurate with their recollections. You compare their story to the body of evidence as a whole, preferably some form of physical evidence. One witness telling the same story as another isn't much of a corroboration. Multiple witnesses can make the same mistake as illustrated by the dozens of witnesses who thought the shots sounded like they came from the GK.
