Love the potential versatility of the Smart/Brown backcourt, or hell even the Smart/Bradley backcourt, but we simply don't have enough scoring without IT in the starting lineup.
For all of his faults defensively and being a poor decision-maker at times, IT is still a ridiculously valuable offensive weapon that is absolutely crucial to our success. The fact that he's one of the very worst defenders in the league due to his size yet still a legitimate MVP candidate shows just how valuable he really is to our offense.
Until we get significant scoring upgrades elsewhere in the starting lineup, IT is pretty much forced to start.
However, I do see (and always have) IT's natural fit as a scoring sixth man off the bench, though. Ideally, Fultz will be ready to take over the starting spot in two to three years, and IT could become the sixth man for us in his last several seasons of his contract. A Fultz, Smart, IT three-guard unit would be pretty dynamic.