During last preseason, ESPN had a segment on Dayne. They pointed out that despite his size, he hasn't done well in goalline/short situations. What they did note, however, is that he has a much, much higher percentage of big plays (15+ yards) per carry the more carries he has in a game. He gets in a grove, punishes the opponent, and seems to see the field better.
What I gathered from that is that he really needs to be a featured back or not play at all. That was the problem with him and Tiki sharing the backfield.