Valid point, I wasn't too specific. Of course it's either look or lean, or maybe a hybrid of the two, whereas trying to look behind the wall results in leaning. So, while you're holding the "lean button" one axis of the mouse controls leaning, but you're free to look up-down.
Also, it's just a matter of habit, I think. For example: player rushes to a cover, taps Q, leans & semi-aims with mouse, and if more freedom for aiming is required, then just releases Q, and he's free to aim, the character won't move, until he presses WASD.
As far as I can see, the problem is simply the fact, that in some situations our two hands are insufficient to control complex motions of our avatars. Maybe with a headband with two infrared dots and a sensor control of the camera could be moved away from the mouse.