It is most likely a Red Herring in relation to the Treasure Hunter Achievement/Trophy.
the description reads (at least on the PS3)
Held every weapon and accessory.
and this guide on gamefaqs says
You only need to have owned every
weapon and accessory at SOME point in time
Now, according to this guide, the Shield Talisman has a Max Level of 2 but no Transformation Cataltst, much like some of the other Talismans.
Also, even if you have completed your inventory you have to go to see Bhakti in Oerba to actually get it as noted in my first link,
To actually receive this trophy you
have to talk to Bhakti, so don't have a heart attack if you don't get it as
soon as your inventory is complete.
as such while trying to get the Treasure Hunter Achievement/Trophy by having some accessories which can go up to Level 2 and not getting the Achievement/Trophy until you see Bhakti the player can be tricked in thinking that they are missing a Transformation Cataltst meant for Talismans when this isn't the case.
As for djsmiley2k's comment, this guide (G0900) says it's only worth if you dismantle started accessories and weapons (Max Level)
It's only worth it if you dismantle starred accessories and weapons, because
that way you get the maximum amount of stuff back. Dismantling may give you
back items, materials, accessories, or weapons.
which for the Shield Talisman you get
- 10x Moistened Scale
- 5x Seapetal Scale
- 3x Crystal Oscillator
weather they are any good to you or not is up to you however alot of the Gil Farming Suggestions don't make use of mass producing Max Leveled accessories and weapons and Selling them or their dismantled components (most are strait farming and selling off enemy drops which you could get from dismantling but be more efficent to get as drops)