This thread is really two discussions at the same time. The first discussion is whether or not Marcus Smart is any good at all. The second discussion is whether or not it would benefit the team with him in the starting lineup. Related but different conversations.
On how good this guy has been and is likely to be... He's 21, has played a bit more than one year in the NBA. It can be argued that he's already the best backcourt defender in the NBA. And yes, he struggles on the offensive end, but sheesh, not many players light it up in year 1 or 2. Give him some time.
I think Isaiah is best off the bench. That leaves Marcus and Avery to start. The only question is whether or not you put Turner or Crowder with them at the SF. I'd actually lean towards Turner, given his ballhandling, and then you come in with a second unit that features Isaiah and Crowder.