I always use a bow at some point on all my characters.. On my heavy armored knightly characters, i try to go Xbow.. Its 'supposed' to hit harder, and shoot further than a bow.. The bows on the Xbow, have more tension on them.. They were actually used to pierce armor back then...Its not used for repeated shots, and its more of a 1 time use thing, or gap closer than someone like an archer, who can fire and reknock an arrow in quick succession... The Xbow used to take tools, or even 2-3 ppl to reload... Some were made with cranks on them to pull the strings back...
So yes, i think the representation of it in Skyrim, is accurate, for a game.. Actually theyd fire far slower!
While i prefer a bow on light characters, knights should go xbow (if any at all)