stop bsing
''Size and weight varied greatly.''
-wikipedia (sourced)
http://www.mercwars.com/shieldtypes.shtml YOU are bsing. Weight varies greatly from 3 pounds to (at the very most, as my source below testifies) 22 pounds. That is a rather large variance.
Shields that I know of from my interest in history that were on any level functional in infantry battle were any where from 20 to 40lbs. The rare few exceeded that. Anything less than that is going to get crushed by a blunt weapon.
Smacking someone in the face with a sheild isn't the same as trying to do the same with two at once, nor is it the same as daggers which require a great deal of agility to do damage.
Obviously your interest in history relies on sources of ill repute and a lack of your own experience.
Take a look at the website linked in the BS. The "Heaviest roman scruta found so far weighed 10kg" http://forums.taleworlds.com/index.php?topic=16941.0 20 to 40lbs? Something smells like a bull on laxatives, to me.
take care to note that the people who say how much a shield weighs who own one states a weight UNDER 10 lbs. Take care to note that the heaviest roman scruta found was 10kg, and the romans had one of the most effective militaries that mankind has ever seen.
Daggers do not require a great deal of agility to hurt someone with. They require someone to be able to punch. If you are fighting with a dagger against someone with a dagger, the first rule of combat is that you are going to get cut. (Source: Personal experience with a medieval combat group, who were teaching people how to defend themselves if someone attacked them with a knife in a modern context)