I think receiving testimony from people not under oath has its place too. When they are testifying under oath, they must be much more careful about what they say. Even to the point of saying almost nothing at all if they're not totally certain which is often the case. This even applies to those who aren't actually trying to lie in any way (many recommend to never talk to police and especially not the FBI without a lawyer present under any circumstances after all). Consider if the members of Congress and the Senate were forced to take an oath before they stood up to debate. Would there be any debate at all?

That said I do agree that _more_ testimony should be made under oath and that we should in general consider testimony not under oath is much more suspect. But I don't think there is a generally correct approach here.