In my inventory, I have a good amount of weapon upgrade cards, however, I never use them because the comparative price to bronze weapon orders is pretty reasonable even when going to a final star upgrade. So, there isn’t much incentive other than to save a little bronze.
I suggest that they be changed so they can be used to upgrade a weapon even if you haven’t leveled up the squad to level up a weapon past a certain point. I believe it would make them more of a commodity rather than a slight discount on bronze order cost.