Flashy or not it doesn't mean that them (Lee/Turner) haven't been better, so far, than the alternative. I think Turner, for example, sucks. But without Thomas on the floor and without Turner on the floor our team has been putrid offensively. Take this to mean, that Smart just isn't ready, so far to be capable of running the team. Hopefully that changes as I want to move away from Turner myself, but that's yet to happen.
On that same angle, Lee so far has been better for us than what Zeller has provided. Hopefully Zeller outgrows this stint, but he's been quite bad defensively for us while only providing very slight marginal improvement offensively.
I don't disagree with anything you said. I agree that the team needs Turner, still. I hope that changes.
As for Lee versus Zeller, I maintain that Zeller hasn't gotten enough of an opportunity this year. He started the season playing next to Lee, which was a horrible fit for a number of reasons, and has never really gotten a chance to settle into a role as a rim runner and occasional spot up shooter / passer that he excelled in last year.
If Zeller has the chance, for at least 5-10 games, to play 15-20 minutes in lineups where the other "big" is a stretchy / mobile guy like Jerebko, Crowder, or Olynyk and he still doesn't really stand out, then I'd be willing to move on.
I also think in a big-picture sense it's better to play Zeller. Lee is here for a one year pit stop to rebuild his value before he hits free agency. I don't think anything he can do from here on out is going to really open anybody's eyes about his current value. So, mission accomplished for him and his agent (except that he failed to really carve out a major role as he probably wanted).
Zeller is a restricted free agent and could very well re-sign here for reasonable money for the foreseeable future. He was a useful piece last year and I expect that he could continue to be a useful piece for another 4-5 years. Productive, competent seven foot centers don't grow on trees. If the team no longer needs him at some point, there will probably be a team out there willing to give up significant value to get him.
For all of those reasons, I'd much rather give the time to Zeller.