Err lol. It would put more stress on the need for materia. It's a good idea, but, it would be totally out of balance as the price of materia would sky rocket. Besides you would totally take the unique part out of Novus and throw it on everything else therefore making the relic weapon less valuable.

We don't make the weapons we plunder, they are crafted by someone/something else. However we do craft our relic weapon with our own hands and it makes sense for us to be able to personalize it to our likeness. Be happy we can do this much to a weapon..