You mean things like Zenith Medals and Cash-shop items? It's not a bad idea but it may be a little difficult to manage. It might need to lose items with updates (such as the skill card dungeon update making skill cards droppable). Then again, such updates are pretty rare for Onigiri. Up to you.
It's like it was planned to be put in the game then was abandoned. Ordinarily, I don't think it'd even make it onto a normal wiki. I guess categorizing them together may be good for a comprehensive wiki like this one.
Edit: Nevermind. Special refers to the rarity so it makes sense to have them for friendship items. I'll look into the item template because things like Friendship PayAce Vida, much like Stat Resets, don't really need rarities.
Edit 2: On second thoughts, why are they even classed as materials? I'll be changing them to etc. items when I get the chance.