One, like Zero said, I don't think anyone wants to give up that Cleveland #1 pick next year. So I don't know why that's even being discussed.
Two, ABQ's point is fine. Look at the Colts -- they had to let James go, so they just spent a 20s draft pick on a new RB and boom, he's just as good for a fraction of the cost. So sometimes it does work out to go with the young, cheap guy instead of the expensive vet.
But as to why a team would go with LJ instead of the two-back system that gets similar stats, I guess the answer goes beyond numbers. I think that when a defense knows they're going against a real stud of a player who can mow them down all day, I think it has a psychological effect on a defense.
Now, the player has to be a truly great player to have that effect on a defense, but IMO Johnson is that rare kind of back. Maybe others don't agree, and that's perfectly fine. But I think seeing Larry Johnson in the Cowboys backfield in the middle of a grind-it-out drive in the fourth quarter has a lot bigger effect on the Dawkins and Urlachers of the world than when they see Marion Barber or Julius Jones back there.
Not to mention the similar confidence that an O-line instantly gains knowing that they're blocking for a back like that.
Odds are huge that this will never happen, but it's pretty good late-June conversation.