Is there any other purpose for boss souls in Dark Souls 2?

I'm wondering if there is any other purpose to boss souls in Dark Souls 2. I already know about Straid and Ornifex providing weapons/spells/armor in exchange, as well as the aspect of getting a lot of souls for using the souls.

As a primary caster, I don't need the weapons, so I'm planing on consuming all souls except the ones tradable for spells.

Are there any other purposes for boss souls than getting weapons/armor/spells and acquiring souls?

Solution 1:

As mentioned by @Fabian the weapons he linked are all the weapons/spells you can get.

However you also have the option to get unique items if you give multiple boss souls

Straid

Talk to him after trading 4 boss souls and he will give you the Black Robe Set, which requires at least 16 Int to equip. The hood boosts your spell casting speed.

Weaponsmith Ornifex sadly doesn't house the same option.

Solution 2:

Soul of a Giant can be used to make one of boss fights easier:

The more Souls of a Giant you have in your inventory, the less damage resistance King Vendrick has.

However, this soul is not strictly 'boss soul'. But it still drops from one of bosses.