Having the shield being equipped to your back means you can't use it right away in combat (meaning it's just being stored), unless there's a button that swaps your left and right equipment slots with them.
This means the back slots is actually two slots, for the left and right hand.
Otherwise, It can stay as a single slot, and can be swapped with the left hand with a touch of the button, but the right hand wouldn't swap, enabling you to swap items in different combinations.
Just because its a possible additional feature, doesn't mean you need a definite negative effect from using it.