1/ Letting the opponent prepare for two different quarterbacks even if Buchner isn't going to play
2A/ The bowl game is a chance for Notre Dame to began assessing their QB situation for next year. Buchner hasn't had much game experience, and in the 2 games he played in this year, both the offensive line and receivers were in the very early stages of development, to put it mildly. If they can get a look at Buchner in a game situation with these much improved units, they might have a better idea as to what they have at QB going into the spring which may impact what they might consider with regard to bringing in a transfer.
2B/. As much as everyone is screaming for them to look to the transfer portal, I am not sure how feasible that option is. How many non graduate QB's are out there who would meet eligibility criteria at ND? Where have undergraduate transfers they have accepted come from? Northwestern, Stanford, Ivy League Schools? Do you think they are going to accept a transfer from some kid from the SEC??
How may graduate transfers in their last year of eligibility are out there who wouldn't want a guarantee that they could get somewhere else that they would start? If there is anyway that the coaching staff can get a better idea fo what they might have in Buchner, it would significantly impact how urgently they may feel they need to lure someone from the transfer portal and then navigate the process of getting them through admissions.