In Gelbard v. United States, 408 U.S. 41, 92 S.Ct. 2357, 33 L.Ed.2d 179 (1972), the Court noted that "contrary to the Government's assertion that the invasion of privacy is over and done with, to compel the testimony of witnesses compounds the statutorily proscribed invasion of their privacy by adding to the injury of the interception the insult of compelled disclosure." Id. at 51-52, 92 S.Ct. 2357.