The Clans need the position of IlKhan as much as our societies need police officers. ;)
The various clan watches, are the police officers of the clans. And clan laws are only for the civies; for warriors: you can do whatever you want, break any rule, as long as you got the power to beat up anyone that would say otherwise (i.e. ToR/ToG). The Khans are the leader of nations, like presidents and prime ministers; but the ilKhan? He's like the chairman of the UN; he can frown, threaten, but only has power as long as someone listens to him. In the clans, the ilKhan's powerbase is his own clan. Ulric can't tell the bears "Yo dawg, I ordered the falcons to do something, and they laughed in my face! Go kick their ass!". He has to use his own clan as enforcers; and with the crusader block, it's a waste of time trying to. Ulric (and any ilKhan, really) wasn't going to put wolves life on the line, to impose his will on a hostile, crusader clan, just so that said clan would be more successful; that would be silly. Clanners would much rather let other clans to eat what they're cooking, and exploit the opportunities that open when they fail spectacularly.
Expecting the ilKhan to be some altruistic idealistic fanatic, that would hurt his own clan to help their enemies "in the name of the greater good", is unrealistic. Simply give me one example of an ilKhan that jeopardized the interests of his own clan, to help other clans.
Because there wasn't a need for them to work together for the majority of their history.
I think working together is against their culture. It's not important if the objectives are met, as long as you give a good accounting of yourself. From the individual warrior POV, even if the operation is a complete failure, an individual warrior can still be considered "successful". In the IS it's almost the opposite, as was said "victory needs no excuses, defeat deserves none".
The 90% number is pretty much BS. But either way, any operation would have ended a lot worse without an IlKhan.
It proved that an ilKhan can mostly point a clan at a target, and allow them to complete it "their way"; trying to micromanage them is folly, and even if they went completely against your orders, there's nothing you can do, as long as they were successful in whatever it is they chose to actually do. St. Nick was pissed off at the Jaguars for massacring civilians; but there was only so much he could do, because they were getting shit done.