Everything you said is true, and is known by those who run spy agencies. And none of it changes the fact that the only way to get human intel on these guys is to employ those kind of scumbags. It's up to the people who analize the data they get from them to determine what's real and what's bogus.
Take Aldrich Aimes; a total scumbag traitor, and also a gold mine for the Russians. Too bad the KGB didn't share your opinion.
Your point is well taken, especially with regard to Ames. I just have concerns about the quality of intel you get from these guys, particularly when the agent running that informant isn't alert to the fact that the informant could just be stringing him along.
Actually, the complete failure of Echelon to detect any preparation for this attack does point to a massive overreliance on and overinvestment in SIGINT and a massive failure of HUMINT.