The shield goes back to antiquity, examples of shields date to prehistorical times. It is used and was only used for defense, right? No it was not. The shield was also an excellent weapon, used with skill, it could be more devastating than a sword. Picture a table top rim slamming into your face and you will understand. In the mid to late medieval age shields were reinforced around the edge, not just to block the blow of a weapon, that is what the face of the shield is used for, the edge became a blunt weapon, useful for smashing feet, chins, necks. For circular shields the central boss which was made of metal could be used as an iron fist. The romans liked to use metal covered shields not only for the extra protection, the corners of such a shield are weapons.
Our movies of today are not accurate in their depiction of armed combat, in general, there are a few exceptions, very few. None of them really show the shield as a weapon of great effect. This is because we no longer understand the shield, we do not use them, we have no experience with them anymore. Even a small buckler can be quite deadly, but like all weapons in hand to hand combat, there needs to be training, strength building and practice to make it affective in combat.
I find it revealing that all established medieval re-enactment organizations do not permit the use of the shield as a weapon! None of them. Their insurance will not cover it, they will not condone it. There is a very good reason for this. Being clubbed by a wooden weapon while in metal armor can lead to injuries and bruises, being rim smacked by an 8 pound wooden shield in the same armor can still lead to death. Even groups that use only choreographed fighting balk at shied use other than defensive, because someone will get hurt or killed.
My one concern is that someone playing around with a shield, someone untrained and ignorant of the finer points of combat, will crush their best friend's skull by accident. I have seen crushed hands when someone was "joking around" with a sword and shield, when their friend caught their hand between the shield and a tree. Crushed. These are not toys, they are wonderful to use, great to learn how to use them in combat, but let us be clear, the shield should be listed as one of the most dangerous weapons invented.
(a bit of a side note, I know I may sound harsh at times, but that is because this is a very serious subject and no weapon should be used lightly)