While the exposed belt holding the placard in position might be destroyed by the opponent, you have to ask yourself: is it worth it?
Destroying the belt will have some effect on the knight, since the placard will not sit in its proper place anymore. However, I doubt that the knight will be severly hampered. Reenactors with a suit of armour of this style could easily test this.
On the other hand, if you attack the belt, you are not attacking the knight. This opens you to counter-attack.
So, my guess it that the belt is a low-gain, high-risk option for the attacker. Thus, there is little need for the armourer to protect the belt in any way.