Rarity is never a deciding factor in balance. Giving rares a stronger power will only make the strong stronger. As of now, these weapons are useless to a veteran who has acquired all rares.
Look at this scenario:
We have a card type A. A has a card set B, the buyable cards, and card set C, the rares. And let's say we have a card set D, the other buyable cards.
Following your argument, card set C should be more powerful than card set B because they are rares. So let it be.
Because card set B and D are all non-rares, they should be balanced with each other. And so it happens.
Now you get shards! SHARDS! Shards everywhere! Shard of Focus, Patience, Divinity, Freedom, Integrity, Wisdom and other OP crap. And no, they are rares, they should be powerful! They must be strong! Congratulations, you just ruined the game.Well that escalated quickly.