You hit on the reason. With Shelley in for a couple of plays forces other teams to account for him. He is different than Huntley so now there is that to think about. If they neglect that prep they risk Shelley having success and Utah going that way until he is stopped. It takes time pre snap to notice.
Other benefits, if Huntley goes out, Shelley will be better ready. Next year it will be Shelley that will battle Rising for the starter spot, he needs to be the best possible.