I think it is a good idea. He fills a couple of roles.
He is an offensive capable player to come off the bench. This helps the Celtics biggest weakness.
He has a legitimate big man size that can help the Celtics defense against the bigger teams. While I believe the Celtics would still beat these teams (76ers, Miami...) in the playoffs, having the extra size could make it easier on the Celtics and on Horford's health through the playoffs.
He can grab rebounds. Can never have enough of those.
He is a vet.
And finally, he only costs the injury exception.