Troy, you pretty much said everything I was thinking.  
As much as we love Grace and know she's capable of handling anything thrown at her, she is still a relative unknown to his fans.  We're used to the casual jam band way of doing sit ins - maybe that isn't as accepted in Chesney's world - and who knows how much time they may have had to rehearse anything.  The man seems to know his business and to be interested in mentoring Grace - guess I'm content to wait for Red Rocks.  A verse would have been nice as KC said though . . .