I don't think that overlap in styles is a problem (we do have the Odenta on DRK, which is functionally a giant SAM weapon, and let's not forget PLD's Zantetsuken), but there is some need to make the weapons from the same instance or set look distinct. That's where the problem lies. As an example, if you were designing an ice-themed Gunblade, do you just copy over the PLD weapon and add a trigger to it? It's a bit harder to make visually distinct than say for DRK, where you can just take the PLD weapon and double its size.