It honestly depends on the raid tier, this raid tier we are currently on is probably one of the least involved for off tanks, but that still doesnt mean theres not responsibility.
E5s: Raiden "Add", invulning a centaur charge
E6s: Forced Tank Swap Buster (although the the original tank can take aggro back almost immediatly after)
E7s: Randomly targetted tank buster and add phase kiting
E8s: Forced swap busters and auto's targetting both tanks in some phases
So even if there is less to do than other tiers, theres still a fair few off tank duties, espeically if you're supporting the MT with aurora and such.