It looks like both of the authors are academics with no actual experience in intelligence work. Considering how much intelligence work is classified and non-public, their conclusion sounds like the equivalent of claiming that there is no proof IT people are necessary because your computer works.
That's whatever the opposite of "appeal to authority" is. For starters there's a lot determined researchers can learn from declassified information about old ops and public information about current budgets and so on. And even if they worked in intelligence they wouldn't be able to publish anything classified.
What intrigues me is the claim "intelligence work is never useful". This is easily falsifiable with Wikipedia, which I assume even university professors deign to glance at on occasion. It's such a weird statement to make I feel like there's either more or GGP is mis-remembering something important from the book.
>Military historian John Keegan called it "the most stunning and decisive blow in the history of naval warfare",[9] [...] Most significantly, American cryptographers were able to determine the date and location of the planned attack, enabling the forewarned U.S. Navy to prepare its own ambush.
Best of all: it was (partially) a chosen plaintext attack!
> Admiral Nimitz had one critical advantage: US cryptanalysts had partially broken the Japanese Navy's JN-25b code. Since early 1942, the US had been decoding messages stating that there would soon be an operation at objective "AF". It was initially not known where "AF" was, but Commander Joseph Rochefort and his team at Station HYPO were able to confirm that it was Midway: Captain Wilfred Holmes devised a ruse of telling the base at Midway (by secure undersea cable) to broadcast an uncoded radio message stating that Midway's water purification system had broken down. Within 24 hours, the code breakers picked up a Japanese message that "AF was short on water". No Japanese radio operators who intercepted the message seemed concerned that the Americans were broadcasting uncoded that a major naval installation close to the Japanese threat ring was having a water shortage, which could have tipped off Japanese intelligence officers that it was a deliberate attempt at deception.
Tangible and in the national interest yes, but not necessarily in the interest of the general public. E.g. the interception of an offer from Siemens (Germany) for a high speed rail system for South Korea intercepted by the French intelligence, see https://www.europarl.europa.eu/sides/getDoc.do?type=REPORT&r... (look for TGV)