As it currently stands neither Agility or Dexterity are useful for Magicka-based Staff users. Agility increases weapon damage and Dexterity increases critical strike rating with physical attacks (weapon critical). As staff damage scales from max Magicka and spell damage neither of these are of any use to Magicka-based staff users.
Any reference to "weapon damage", "weapon critical" or "physical damage" refers only to weapons that deal physical damage (everything but staves).