I know that sounds kinda weird but think about it. The mind of a Psion is unparalleled. In Cayde's words, they were able to "tangle with rasputin." Rasputin is a Warmind. An A.I. programming that was built to win wars and defend the human race. Obviously he didn't do that, but he was still designed to [i]win.[/i] Which is.. technically what he's doing. Cabal are a highly militarized race. Everything about them deals with wars and conquering. What better candidate to call the shots and make military decisions than a ultra intelligent Psion?