Board Thread:Questions and Answers/@comment-1031179-20161109052033/@comment-6697950-20161109181659

The main reason you'd want a weapon skill is to unlock Sword Skills for the weapon. Skills also have mods that can provide either a decent bonus or new abilities (e.g. Quick Change, Tracking etc.). Having the Shield skill apparently grants a taunting technique, so I assume that even armour might have bonuses like that. Otherwise, having a skill isn't mandatory.