Any reason to not use weapon parts that increase rarity?

[tiny violet]

I just hit 200 of a weapon I don't really care about, should I use it or is there another use for these items?

[Jasgo]

The only other use for weapon parts is trading them for Weapon Medals that in turn can be exchanged for Grindstones.

Definitely better to upgrade the weapon. Even if you don't plan on attacking with it, its R abilities could come in handy at some point.

[MjX]

In time it will be useful, there isn't a weapon that isn't.