Itachi did not have any knowledge of kirin because sasuke made the jutsu by himself (due to his intellegent and skillfulness) .However when sasuke was about to use kirin he give an explanation of the jutsu speed and power before he launch the attack.Itachi at a last minute decide to use his best jutsu to counter it(susano).If itachi did not know about kirin powers and speed he would have die /vapourize.
I will say kishimoto make itachi live just for temporary plot shield to let the views know itachi back ground and his true motive against the uchiha clan.Saving Itachi make tobi or obito explain itachi story and this also help sasuke have a change of heart toward his brother and to some extent konoha.