He does offer some thing the other 2 CBs don't. I can see advantages when we need someone to provide defensive cover but also drive from deeper positions or add options to midfield when we're needing a push. I can also see him at full back, in that they're inverted anyway. Definitely good to have in the squad.
And there's the qualities given above he brings to the team anyway.
However, if we had a full strength team, doing well, it would be difficult to start him. But he's versatile, and we have developing players whose form can dip, or injuries like Downes. So plenty of starting opportunities, but not necessarily every game.