Shields in general tend to be rather underwhelming for enhancement. From a defensive standpoint, enhancement shaman (and shaman in general) don't get the itemization or talents that enable them to get much benefit from shields. If you look at classes like paladin and warrior, they get all kinds of talents that increase their chances to block, stats like STR increase their block value, and they get gear for prot specs that further enhance the benefit of a shield.
Shields are typically worn by elemental and resto shaman because they can't dual wield and their aren't too many 2h axes/maces with caster stats.
Blizzard has caster shields in the game specifically for holy pallies and ele/resto shaman.
As far as solo enhancement goes, the choice to use a 2h/dual wield vs. 1h + shield revolves around the same reasoning as the choice to use Stoneskin Totem or Strength of Earth Totem...the defensive option might let you withstand a bit more of a beating; the offensive option makes you less likely to need that extra time ;D For enhancement shaman in groups, you'll never find a shield with the kind of stats that would boost the damage output of a single 1h weapon relative to the kind of damage you can do with a 2h weapon or, ideally, dual wield.