You can use the iPad in any orientation you want so the home button is not always at the same place. Having to visually locate the button, remove your fingers which are touching the screen, relocate it to the button, and single or double press the home button, sounds trivial, but it's a more conscious process than simply 4 finger pinching to close the app. Also the multitasking gestures give us better feeling of the direct manipulation of the UI.