Normal damage are actually the worst kind of damage because you don't get any bonuses. Changing Antigua the way you've suggested would actually nerf it. Normal damage are good for new players who are still learning the ropes and has a very vague idea about damage and monster types. But that's it. The more advanced players price Piercing/Shadow/Elemental above Normal damage.

Normal damage, as well as swords, are for simple-minded people wo like simple ideas. So it's good that Sealed Sword does normal damage because sword users are not able to handle more sophisticated ideas ;-)

Antigua does Piercing damage and can be upgraded to deal additional Shadow or Elemental damage. This gives player much more options to deal extra damage and improve one's efficiency but also involves more sophisticated tactical thinking. But guns generally involve more thinking so it's ok.

Please read a post below about Antigua. If Antigua had Normal damage instead of Piercing, the combo mentiond in this post would be useless. And that would make me sad panda :-(

http://forums.spiralknights.com/en/node/2252#comment-13951

Disclaimer
;-) <- this emoticon at the end of the statement means I'm not entirely serious about given statement. So don't be offended.