I hadn't got around to watching the BBC documentary The Power of Nightmares until recently, but it brought up something relevant to my earlier discussion of intelligence agencies: outsiders' desire for relevance and insight.
The documentary discussed (among other things) the idea that the Soviet Union was supporting terrorism around the world back during the Cold War- an idea championed by neoconservatives as proof that the Cold War really was a Manichean conflict between Absolute Good (the United States) and Absolute Evil (the Soviet Union), and that the forces of darkness were intertwined in everything they did. They thought they had proof of it in Claire Sterling's book "The Terror Network". Unfortunately, Sterling's "proof" was largely the result of CIA black propaganda- that is, information that is covertly published by an intelligence agency that purports to be from its enemies.
This highlights a major problem: outsiders want information, and can in turn gain the ear of the powerful. Even if an intelligence agency isn't ideologically driven (or isn't ideologically driven enough for outsiders) the simple fact of secrecy and the difficulty of analyzing intelligence can lead to enormous problems, because outsiders can take what information is available publicly and draw wildly erroneous conclusions from it. This is especially likely when they're looking for proof of something, rather than trying to interpret the data on its own terms. If intelligence agencies are manipulating the openly-available data, then the whole thing can become disastrous, because said agency simply cannot correct the outsiders on their mistakes.
We also saw this with the "stovepiping" of intelligence to the White House in the Iraq runup, and that showed an interaction effect that can exist. If outsiders gain the ear of politicians and convince them that the intelligence is wrong, this can provide a powerful unstated incentive for intelligence agencies to walk the line, an incentive that the agency and its analysts will obey in fear of the risk of losing their relevance. This is probably what happened at the CIA; aside from the direct pressure that almost certainly existed (but which no career-minded professional would EVER admit to) there would be this powerful indirect pressure. Analysts aren't superhuman. Maybe not all will bend, but enough will bend, and the individuals who say "no" will not speak for the organization.
As I said, intelligence agencies are a dangerous but necessary tool; covert ops are even more so. The most dangerous actor, however, can be the outsider with an axe to grind and an ear to bend. At least intelligence agencies are supposed to speak truth to power. Richard Perle sure as hell doesn't need to.