But then you'd have a currency menu* "bloated" with useless currency items because they want to add a currency for every little detail to avoid abuse. That would just shift the bloat elsewhere. Plus there is also their steps in which you have to "physically" turn in the items. Since you can't upgrade the items while equipped, you need to have them in your armory chest/inventory for said turn in. So it seems that the x crystals + weapons is another "system limitation".
*that is another horrible thing I have learned to tolerate, but I digress.