Originally Posted by Graul

Rage Winterchill, Anetheron, Kaz'rogal, Azgalor and Archimonde ALL have CC or AE that can, and often do take you out of combat for brief periods of time throughout the whole fight. Sometimes you can get lucky with them and barely ever have to move, other times you are not so lucky. Najentus has his shell, which negates damage for a couple of seconds until your raid heals up and you break it. Not everyone has melee chase Supremus around during the kite phase, although it's possible, but there are still plenty of times you have to be away from him other than just that. Mother has her random ports (which you can get hit with zero times, or multiple), Council has rough AE quite often and with Illidan, it's just a matter of the phases. There are PLENTY of examples as to why you will never be taking advantage of a Mongoose proc 100% of the time, because you will never have it proc as soon as you hit a target for the first time, then fade right before you have to leave on even close to a consistent basis. You also do kill trash during the raid, and while many scoff that trash matters, it does. Otherwise the whole raid could just afk teleport to every boss.