Firstly, it's elemental defence, not elemental resistance. Statuses (shock, fire, poison, stun, freeze, curse) have resistances, whereas damage types (normal, piercing, shadow, elemental) have defence; you can be completely immune to a minor status (statuses inflicted by haze bombs or zombie breath), but you can't completely reduce damage taken to zero.
Secondly, shields only work when you have them activated. It does not provide any protective bonus if you're not using it directly (although some shields may have offensive bonuses instead, such as Swiftstrike Buckler or Barbarous Thorn Shield).
Thirdly, the Dragon Scale series don't have any normal defence, so they will break much more easily than most other shields in the majority of situations, since most attacks will do some normal damage.