Consistency of base mitigation abilities for mobs allows for a consistency of combat, while still allowing interesting and engaging offensive abilities, differentiating defensive abilities, and various mob characteristic design. It's a bread and butter ground work that keeps mobs consistent in their ability to mitigate damage from all players. It can definitely create some weird situations where a grub really shouldn't be able to parry, but it can.
Drysc goes on to suggest imagining that the grub quickly spins a silk sword to parry your attack. Similarly he suggests that moths parry with their "Fel Proboscis".
So, it seems we should expect all mobs to be able to parry and block us while we fight them and to daze us while we run away, regardless of their species or appendages. It's a suspension of disbelief thing. We are supposed to rationalize or ignore how these creatures are doing these things and concentrate on the fun aspects of the game. Immersion into Azeroth suffers from balancing the gameplay experience.
I would like to see more customization of the enemies we fight -- less consistency of the mobs and more classic RPG battle rules, like skeletons being susceptible to blunt weapons and bleeding attacks not working on creatures without blood. On the other hand, I've had more fun playing WoW for a longer time than any other MMO. Perhaps it is just the classic geek conflict: purist vs. fanboy (or fangirl in my case).
Does the fact that grubs can parry bother you? Would you prefer less consistency and more believability? Do you think Daffy is better as Robin Hood or Friar Duck?