Gobuske kept his spriggan on the defensive team for a multiple of reasons. First, though it is one of his most intelligent monsters and can even speak fairly decent common tongue its not that great a leader. His hob goblin commanders were far better in this regard. Second, it's fairly slow, not the best trait for an offensive team. Third it had quite powerful beastlike instincts, prone to bouts of aggression wherein it would forget its orders and surroundings. Frost would surely have figured that out after a few attacks and easily dragged it from the defence of the main assault. Much better to have it wait and get the enemy to come to it. And with this successful attack, he felt that he made the right call.