Various well-reasoned comments on my post yesterday challenged my suggestion that the courts might defer to the judgment of Congress that executive privilege should yield in any particular case on the ground that Congress, as a politically accountable body, may be better situated to balance the need to override the privilege and the executive's need for the privilege. If executive privilege is rooted in separation of powers, the objection goes, then deferring to Congress in such matters means that the fox guards the chicken coop. At the very least, the objection goes, there is no reason to prefer the judgment by Congress that the privilege should be overridden to the judgment of the President---who is also politically accountable---that it should not be overridden.
I have two thoughts in response. First, I did not mean to say (nor, on re-reading what I wrote, did I actually say) that this alternative, defer-to-Congress, approach is actually right. I only raised it as a possibility to call into question the conventional wisdom. As I noted, the idea wasn't even mine originally.
Second, in defense of deferring to Congress on the matter of executive privilege, it's worth noting that although the Supreme Court in U.S. v. Nixon does say the privilege is constitutionally rooted, nearly all other privileges have the status of sub-constitutional law: attorney-client, clergy-penitent, etc. The principal exception is the Fifth Amendment privilege against self-incrimination, which, after all, is stated expressly in the Constitution. Executive privilege is at best a structural inference.
Accordingly, were the Court writing on a blank slate, executive privilege might best be characterized as a matter of federal common law---i.e., judge-made law that serves constitutional values but is not required by the Constitution and can be superseded by Congress. By comparison, the federal privileges applicable in federal court (with respect to issues as to which federal law provides the rule of decision) are, per Federal Rule of Evidence 501, matters of common law.
To be sure, the analogy to federal common law is not perfect. If we were to treat executive privilege as a true matter of federal common law, then it would require a statute passed by Congress and signed by the President (or passed by 2/3 majorities in each house in case of a veto) to supersede the privilege, and we would expect the privilege to be superseded in general language---not by a committee of one house for a particular case. Nonetheless, the uncertain constitutional pedigree of executive privilege and the sub-constitutional status of other privileges combine to make other sorts of deference to Congress on these matters a plausible alternative to direct judicial balancing.