Like Kyle Singler?
He wasn't projected 3-8. It was more 9-16, according to what his parents said after he made the decision to comeback. If he were projected 3-8, I guarantee he would have left for the draft. I bet he still ends up drafted in the top 16.
And he wouldn't have been anywhere near ready for the NBA, not that he is right now. What's the saying? It's about the second contract, not the first. You don't want to be out of the league after your rookie deal. Don't be shortsighted.
Someone like Kendall Marshall made the right decision to leave when he did. Does anyone honestly think he would have fixed his major weaknesses in college to greatly improve his draft stock? Even if he managed to do so, at best he was a late lottery; which is the position Phoenix picked him up at, anyway... He wasn't going develop Lawson footspeed's or Lynch's defensive ability next year, the year after, or 50 year from now.
McAdoo, on the other hand, is far from a finished product. He wants to put the ball on the floor and attack, but he can't dribble very well. He wants to shoot, but his jumper is slightly above average AT BEST. Usually it looks worse. He has an idea of his game, it's a plausible way to develop considering his skill set, but that very skill set is not developed enough at this moment to bring out in a college setting.
Also Dex wasn't a 5 star, fwiw. He was when he was projected to be a PG in his class. He didn't end up one, dealt with an injury his senior year, and dropped to a high 4.