Currently, a shield provides an armor rating increase no matter which hand it's held in. However, it's only possible to block attacks when it's held in the left hand, so one could hold a shield in the right hand and still gain the additional armor rating to help successfully dodge attacks.
>glan
You glance down and see that you have nothing in your right hand and a rune-covered spiked shield emblazoned with a towering giant in your left hand.
>inf
Armor rating: 48 (without armor: 21)
>swap
You move a rune-covered spiked shield emblazoned with a towering giant to your right hand.
>glan
You glance down and see that you have a rune-covered spiked shield emblazoned with a towering giant in your right hand and nothing in your left hand.
>inf
Armor rating: 48 (without armor: 21)
>wear shie
You sling a rune-covered spiked shield emblazoned with a towering giant over your shoulder.
>glan
You glance down at your empty hands.
>inf
Armor rating: 40 (without armor: 17)