Young's value in the NFL is most likely at it's peak right now. Playing another year in college would just open up the possibility of his value slipping.
Leinhart is a good example. If he had went pro last year he would have been the #1 pick and would have been paid as a #1. By staying in college, he cost himself a year of salary plus any money lost for not being a #1 pick this year.
As far as picking up the heisman, that used to be a huge reward, but it's really not anymore. It certainly doesn't add any value at all to a player when NFL teams look at a him.
If a player is predicted to go in the top 10 or so, they should always turn pro. The NFL has a service that informs underclassmen on what their probably draft position will be. That prevents players from being screwed my incompetent agents.
Don't worry Matt Leinart is going to do OK. Many of us wish we had his problems.