one class melds all is still a bad idea. whether you look at the materia melders description or not. it still makes no sense that a weaver can meld materia to a sword.

in the land of examples here's one. mr weaver guy is asked hey can you put this materia on my sword.
sure says the weaver. now where does he meld. he knows nothing of swords or where the most suitable place to attach the materia is. does he put this materia on the handle thus preventing anyone from holding it properly or does he put it right on the tip of the blade effectively blunting the pointy end. or the same example with a helmet maybe mr weaver puts the materia on the visor thus obstructing the wearers vision

you don't just give someone a screwdriver and say attach this shelf to this wall. any one can use a screwdriver but do they know how to make sure the shlf is level or secure or do they simply screw it in and hope for the best.