You rest players when you know you are a team that is going to go deep in the playoffs, especially if those players are getting on in years or nursing injuries. The Celtics are neither old, nursing injuries or expecting to go deep in the playoffs. The only way they DO go deep in the playoffs is being healthy and working on all cylinders as a cohesive unit with known rotations and roles and giving every effort.
So they gotta play and play hard for the rest of this year. There is no doubt about this in my mind. Rest is for the offseason.