He comes equipped with the Defender Path. In the skill tree is the ability to become proficient with Plate Mail. I assume taking this trait has no benefit whatsoever for Tuatha?
I would propose then that if a Hero unit is purposely unable to use a Trait (like above), that the Trait in Question is "Blanked Out". Such that if the Trait is "blanked" it can be freely skipped over on the next trait selection.
In Tuatha's case; coming from Defender II, his next option in the tree would be either Arching Shield Bash or Invincible.
Unit Trait advancement is already really slow, let's not make it worse.