The Simbiari are a race of uptight technocratic assholes, given Proctorship over humanity for decades to weed out the people, religions, and cultures most antithetical and dangerous to the Milieu and to bring overall human law and outlook into line with the Milieu. The Simbiari also have a case of raging jealousy for Mankind's potential and apparent favored status, deplore Humanity's primitive ethical evolution, are outraged at their premature admission to the Milieu, and are completely unable to comprehend Man's relatively freewheeling and lackadaisical mental outlook - causing them to overreact to humanity's flaws in the most totalitarian ways. Later in the books, the Simbiari Proctorship is acknowledged as unsuccessful and a near disaster, turning a large portion of Humanity against the Milieu (who would later go one to become the core of the Rebellion). Realistically, any other race would have been better as Proctors: the Krondaku are stolid and unflappable, with two successful Proctorships under their belt; the Poltroyans are similar in evolution and sympathetic to Mankind, with only ill luck preventing them from having one successful Proctorship; and the Lylmik are the founders of the Milieu, and while decidedly weird, are perhaps the most dedicated to raising Humanity to full coandunation (the Gi, on the other hand, acknowledge themselves as too flighty and sex-obsessed to ever be considered for Proctors.) However, the harsh treatment of Humanity by the Simbiari are what caused the Rebellion, and their reproductive statutes are what caused the birth of both Marc, and the violation of the same are what caused some of the specific circumstances necessary for the birth of Jack. Without Marc, and the rebellion he was to lead, and without Jack to successfully oppose Marc, Humanity would never have achieved Unity during the Metapsychic Rebellion, and Marc would never have entered Exile, eventually become Atoning Unifex, and start the process that would eventually lead to the formation of the Milieu.
Every sentient species to turn up in the galaxy that does not evolve to be metapsychic or fails is judged unsuitable for membership in the Milieu by their proctors is quarantined in their home system and doomed to slow extinction as they run out of resources (unless the failed proctorship caused the race in question to wipe themselves out in some kind of war). There have been hundreds of attempted proctorships. The number of species to achieve full membership in the Milieu? Six.
It's strongly implied that once Humanity is fully Unified, the Milieu will have enough power, skill, and empathy to go back and raise all those quarantined species up. This may in fact be the reason that humans are so important in the scheme of things.
FOUR BILLION PEOPLE died in the Metapsychic Rebellion. Well, two billion humans and two billion Krondaku, but the point is the same.
Even worse, while the human deaths were a mistake (a planet-buster missile launched at a ship and lost control of), the Krondaku were deliberately murdered by the Rebellion. And the murder was effectively televised to every psychic in the Milieu).