Well, it does take two hands to operate a bow. It's hard to imagine holding the bow frame and the arrow with one hand and aiming at the same time. Just from a gameplay mechanic, you can't attach a buckler to your arm, like you could in real life (isn't that how they work? they strap on to your arm?)- they always require it takes a hand.
I still think the fifth class will be a bow user, but that doesn't discount shields, in my opinion. You don't really need a class to epitomize shield usage.