The Hiss case has been the liberal cause celebre for the last 50 years. If they're re-writing of history on this case is accepted as "correct", they will argue that their "interpretation" of Cold War history (e.g., the commies were never that big a threat, Reagan was a war-monnger, McCarthy was a drunken fascist, etc. etc.) is and always has been correct. The Hiss case has always been a great embarrassment for them.