A shield isn't just for defense like armor, it can also be used as a weapon. You can effectively bash some one down for a killing blow with a shield, and some cultures who used lighter shields or shields strapped to the fore arm would give them a spike on the boss to stab with. Also, berserkers aren't necessarily trying to just show their bravery, they can also be opting for mobility. In that sense, a light round shield doesn't really staunch mobility, and provides some protection in addition to being useful as a bashing side weapon.
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