I'm currently a level 56 Demon Hunter with a crossbow and quiver. The best quivers I've seen give significant damage boosts, although with my bow it's not necessary (able to kill normal mobs relatively quickly on Hell difficulty). I've read that using a shield as your off-hand, typically something high in dexterity/vitality, is beneficial since Demon Hunters aren't the strongest class. While solo-ing I get low on health frequently (although I regain it pretty quickly due to near-constant crits and lifesteal), but in group play I'm almost always out of the line of fire.
Long story short, why (and when) should a demon hunter use a shield?