Even in a keeper league I wouldn't touch him until a little bit later on. I might spend a 7th-9th round pick if I was really high on him and/or I had LT but I don't think I would reach much higher than that. You have to keep in mind that this guy is still unproven. He has potential, of course, but he hasn't proven anything other than that he can be a great backup to the best RB in the NFL with a great offensive line and a fantastic fullback. What will he do on his own, on a different team, in different circumstances? Who knows....that's why he's not worth that high pick.
You never know, he could end up being the next great fantasy RB in the NFL....or he could go to another team and we'll all realize he can't carry the full load for a team on his own. I wouldn't mind having a guy like him stashed away on a dynasty team, but in a keeper league? He'd be a stretch to keep in most leagues even if he gets his own gig in a couple years.