Here is another angle. Ainge has never really valued the PG position in building a championship team. When he acquired KG and Allen to pair with Pierce he went to bat with a defacto PG in Rondo who at that time had the job of defending and bring the ball up over halfcourt, initiate the offense through Pierce or KG, then stay out of the way on the weakside. Ainge also never acquired that backup PG for years instead making due with House and partial years of Cassell and Marbury all on the cheap.I think one way you could view how the Celts view IT is that they really have no other real PG on the roster. Avery Bradley is a great player to have on your roster until he becomes too expensive. You don't pay high end money for a defensive oriented 2 guard imo.
Edit: Smart and Rozier are listed as PGs but that seems more in name only rather than true PGs
With the addition of Hayward the offense doesn't necessarily need a traditional PG to function. I can see Smart performing in a similar role as Rondo early in his career if Isaiah wasn't here moving forward.