The problem specifically is that the damage does not scale with the unit group size, but the cost of equipping them with it does. This makes it one of the worst items in the game to use on your unit designs.
So what you are saying is that everybody in the army is equipped with throwing knives, but one person gets to throw them... I can see the order now...
"You in the front, George is it. George you are the only one allowed to throw a knife."
"But sir, we all have knives, can't we all throw them at the same time?"
"No, and I don't care if everyone has a knife, only George can throw them."
"But he has a bum arm and can't throw them very far."
"My order still stands... George is the only one that can throw them and that's final."
"Man I should have signed up to be a mage... they get to at least use their weapons."