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So sure you could have a clip on mini shield with a defense bonus of 10 % but what if your accuracy becomes worse???

Exactly! The penalty for a shield is less combat ability. For melee characters, using a shield means not using a two-handed weapon. So a bow user would be penalised due to increased weight and limited vision. On the other hand, they would have a better chance of survival should the foes come close or fire arrows back at you.