When liking up with Rowen he will cover you if the enemy decides to cast a spell on you. Like the description states it will automatically activate once the spell is cast on anyone in the party, not just you. That said, if you have a party consisting of yourself, Rowen and Elize along with either Leia or Alvin you can set the other 2 AI to "only" under the actions in the strategy menu and have Rowen set to "protect yourself" you can take advantage of a good battle against spellcasting foes. It helps to put the "Minimum Damage" skill (Aifread treasure, second part of the Vicalle Mine) on Rowen as he'll occasionally attack foes, especially while you're linked.
Good foes for this are any form of elemental winged creatures and the goblin class enemies.