The Washington Post has an interesting article today about the Library of Congress' Congressional Research Service which spends 100 mill. in taxpayers money annually to produce "private" reports which are only accessible to members of Congress.
It's an interesting report. The LOC was designed as an information resevoir for the hill, just as the CIA, NSA, and FBI compile confidential, comprehensive information and intelligence reports regarding national security which is "mostly" accessible to the executive and congressional branches of our government.
In the justification to keep the reports conifidential, I think the real question becomes a matter of national security. Would the LOC reports qualify? Doubtful, but anything can be considered a matter of national security if you want it to.
Wired linked to this website in their coverage of the WP story, which is intent on liberating the reports to the publc. Browse through the titles and you can understand why the information is so valuable.