Definitely would love him sticking around. In fact I'd be willing to offer him a similar contact to Crowder's personally (around $7M/year).
Turner still is a high risk player, and that means he is going to get those frustrating turnovers and bricked shots from time to time. But it's gotten to the point where his good plays seem to outnumber his bad plays something like 10:1, and as long as that's the case he is a very valuable player.
Hell, even his defense has been pretty good lately. Not Crowder / Bradley good of course, but he seems to be holding his own.
His versatility is the key point. With the exception of three point shots, Turner can do ANYTHING out there. Rebound, score, pass, handle the ball, defend - anything. If that guy ever develops a passable three point shot, people might start seriously contemplating him being worth his draft position.
Thing is he is still young (25), and to my eyes he is still very clearly improving. That suggests to me he might still have potential. He's good enough right now to be a starter for many teams, I think...but his abundant energy makes him so perfectly suited to a 6th man role.