Seemed like once BC made a decision about a back it was tended to stick.
I remember back in '04 Tierre Green was running quite well for a redshirt-frosh, he had a big fumble against So Miss(on a play where he ran like a madman), and he quickly dropped off the map and into the defense.
In '06 Jackson was definitely the best back we had but didn't get the starting-job til well into the season then had it taken-away when he was injured but could still go because BC wanted to keep Lucky happy.
I can't attest to Glenn's practice-habits but when he has gotten into games he's run well, the best tough-yardage runner we've had in a while. Considering Lucky is mediocre between the tackles I'd say Glenn should've gotten more carries.