The shield is awesome but is underrated in the game IMHO. The bonuses should probably be increased and shields should be utilized as weapons. So herein lies an interested thing, it seems that the shield was not used as often in single combat type situations as is it was during large scale conflicts. Could it be that in mano a mano the shield was not as useful as say, another weapon, a two handed weapon or even just remaining weaponless. It seems counter intuitive but maybe. Shield designs would also be affected by weapons used and styles of combat.
I like this show. Here is one on the shield.
As for the viking round shield as a weapon,
site. This guy seems to know what he is talking about. check out some of his videos on youtube.
I do not think it would be used offensively as a weapon to cause damage because of its large size - it would be trivial for an experienced combatant to dodge and use the space provided to attack with his weapon. This isn't to say that it couldn't be used as a weapon, but beyond reasonable doubt, I expect it would be a secondary concern.
In close-order formation, the use of the shield as a weapon would be problematic to the whole formation.
Roland Warzecha and for youtube