Not every player is the same.. I am sure Aguero is shit at taking corners so he's better off in the box even if he's not gonna get anywhere near it. McCormack clearly has a pretty great corner and is a short arse so why not let him take them? Just because it's traditional to have the strikers in the box for corners doesn't mean it fits all strikers.. You are right though we lacked a fair bit once he came off. Green and Grealish offered loads of attacking movement, but the guy that would have benefited went off to make way for them which is unfortunate.