So the rules for shields state that a model with a bow can use them to shield, but do not receive a defense bonus, and that mounted models receive a defense bonus but cannot use the rules for shielding.

Rivendell knights have both a bow and a mount. At the same time they can purchase shields for +1 point a model. Why is this an option when the only time they can ever use a shield is if they get dismounted, and even then only receive partial benefit.

Online, most discussions and army lists I see say to give them shields, and even list them as defense six.

What is going on here? Am I missing something, or is everyone else missing something, including the guy who wrote the profile for them.

Is there errata somewhere saying they replace their bows with shields? I rule allowing them to drop their bows? Some exception to the rules I didn't notice, or what?