I'm not sure what you mean... If you're referring to M+K on XBOX, no it's not like that. It's just a comfort and preference thing. It does not enable you to cheat in any way, nor does it break the game mechanics. It is literally translating M+K motion into controller motion. You cannot do anything the controller cannot do.
No, M+K is by far and away superior to controller. So much so that they are banned as the advantage gained by using them in an FPS is too great. You can hit button combos on a keyboard that you cant on a controller, and target acquisition with a mouse is way WAY better than with a controller joystick.