I don't see how this is a big deal. He dominates in 1 inning all the time. So because that inning is changed to the 9th inning means he won't succeed? That makes no sense.
Anyway, Thornton is too valuable to become a closer. It is harder to come in with men already on base and have to get out of the inning. Thornton is great at doing that. Jenks would be garbage at doing that. Let the less skilled pitcher stick to the 9th, when they get to start off with a fresh inning and no men on base.