His entire effectiveness as a QB had to do with his legs. His ability to run changed coverages and rushing patterns, kept safeties looking into the backfield rather than at WRs and made coaches put LBs in front of him rather than drop into normal zones. If he stops running and doesn't improve on his passing, those practices will stop, his PER and overall effectiveness as a QB will drop and the BYU offense will struggle.

He has to throw better. He has to run some.

Another way of saying that Hill is a great athlete and you hope Anae protects him is "Hill is a great athlete, let's not use him so he won't break".