I think that IF Jack Coan is seen as a potential starter of the future, he should be played more than tokenly - with the first string offense if four the five remaining games, keeping his redshirt status.
Doing this will allow for the option of Coan starting if he proves better than Hornibrook or if Hornibrook is injured.
I do not see a disadvantage in playing Coan significantly, barring a major injury to him. It will allow Hornibrook to observe which should be helpful for him. It will allow Coan to get regular game experience which will help him. It will allow the staff to observe and evaluate Coan as well as work with him to improve his quarterbacking.
It should be obvious by now that Hornibrook is not destined to be one of the "Wisconsin greats" at quarterback.
Playing him full games doesn't seem to increase his confidence or cut down on his horrific play at times.
The Penn State game would be the logical game to Not play Coan unless he shows enough talent against Northwestern and Rutgers - to indicate that he should become the starting quarterback.
The only reason I can see for not playing Coan substantially is if he already has been ruled out as Not a player who may start in the future.