I can't believe some of you still use the "he isn't ready for the NBA" excuse for why he should stay. That ship has sailed. That is no longer something kids concern themselves with and the NBA has no problem drafting kids who aren't ready to contribute in year 1.
The problem with TL's stock isn't being ready or not, it's that his upside doesn't look all that great. What's his projection? He'll be 21 at draft time with a body that doesn't look able to compete at the NBA level long term. Can he be a stretch 4 off the bench popping 3's? Of course, but is there more to his potential? He isn't super athletic and probably won't be able to guard SF's in the NBA or bang with interior players. He can have a Channing Frye career if he can add good weight and strength, perhaps, but the longer he stays, the less of a chance he'll have of being a 1st round pick. He probably should have gone pro last year and strike while the iron was hot. I think his prospect status is getting dinged a bit this year due to what looks like lesser quickness/agility and a handle that hasn't improved. He's basically the same player as last year, only a year older and with some added weight (that probably hasn't done him any good). What happens if he stays another year? All of that just gets exponential worse for his stock.