Dunbar has proven he can make a serious difference as a change-of-pace running back. True it would be idiotic to keep him in as a blocker, but you can play him for stretches because he is a threat as either a runner or receiver and thus does not key run or pass for defenses. Against a blitz he's a receiving option instead of blocker.
You can hammer his injury history or his two career fumbles in two seasons, but its hard to ignore a 5.0 rushing average and 8.4 YPC. He's limited, sure, but he is dangerous as an outlet receiver or (gasp) by design receiving threat out of the backfield. He's not Reggie Bush, but neither was Bush himself for his first five seasons in the league.
Bottom line is Tony Romo averaged 4.8 YPA vs. Miami which is pathetic-- we're talking Hoyer/Manziel territory. Dallas seriously lacks team speed and needs players like Dunbar to keep the defense from cheating into deep zones and smothering Dez and Williams. Romo's arm isn't getting any stronger, and so players like Dunbar who can add YAC after a short pass are very important to have.
If Williams stays, then it's obvious Randle is odd man out or you dump the FB (yay). Garrett never utilized the position well while he's been here, and so it needlessly consumes a roster spot.