Also enlighten me as to how reasonable it is for a dagger to damage plate.
Daggers have short and slim blade, perfect for piercing things.
Dont forget that while plate armor offers excelent protection, it isnt some kind of adamantium turtle shell. Even plate armor can be pierced with a good weapon and enough force.
I also think that half-swording is pretty overrated. I mean yeah it is a good technique if you want be sure you land a lethal hit, but I seriously doubt that nobody can land a stab that pierces plate armor without using halfswording.
Note: This heavily depends on the armor itself and the physique of your adversary.