His career he has 84 carries for 317 yards, including 14 sacks. I wouldn't say those numbers make him a "mobile QB." Against Reno they didn't use him to run the ball. I can't even remember a scramble he had.
I looked at Jennings numbers and he doesn't look like his legs are used as a facet of their offense. Their ground game has been bad, at best. BYU needs to keep it that way. If they can run the ball, that opens up everything down the field and the base nickel would have to change.