I haven't played IS yet, but he seems to be an interesting creature.
Once he is in the arena the threats he poses should force the opponent to make different decisions to counter those threats as the discussion above has pointed out. Knowing this, then the ForceMaster should be prepared to take advantage of the opponent's changes during the action stage. For example, if the opponent mage is forced to use a quickcast action at the beginning of a turn to counter another threat, then this provides the IS with more opportunities toward the end of the round.
Think of the IS as providing options to change the dynamics of the Action Stage, rather than just the Action Phase.