I think his role right now is mostly Horford's understudy. To be ready to play in short bursts if Horford can't play.
But since he's old like Horford, he probably can't bring that same energy every night if he were to play consistently.
He's the donut tire in your trunk, meant only to be used for short distances, you don't want to put a lot of miles on it.
Also he helps maintain the rotations. If you remember when Scal was on the Celtics in '08 and '09 he started 9 and 8 games. These weren't games where the starters were rested, but games when KG or Perk were out for a few games, even though he wasn't KG or Perks backup. So Scal would go from not playing, to starting and playing 20-30 mpg. This was to maintain the rotations, so guys like Posey, Big Baby, Powe could still come off the bench and play with the 2nd unit and maintain their current role.