The Bruins and Celtics have shared a venue full-time for 70 years now, part of the culture of Boston is going to Causeway to watch both. From that standpoint, it would be a tremendous shame for the Celtics to move out to their own place, and I think the Garden would be lesser for it (I wonder if, given that scenario, the Jacobs would try to lure the Fleet in, or maybe move the P-Bruins up to fill in dates). By the same token, Boston is somewhat under-arena'd. The Garden is the only arena in the metro that has more than 17,000 seats - Worcester has ~13,000 and Hartford has ~15,600 for basketball. There's an argument that another arena would be a solid financial play, though it would hurt the Garden's schedule and almost certainly pull concerts away from Worcester.
Selfishly, if the Celtics could build a new arena, with the floor on solid ground, and enough collapsible seats, we could have the first Monster Jam in Boston, which I'd definitely be there for.