a problem for high schoolers that directly go to NBA is that they are more able to have health problem. NBA is far more physical with big men so if they have problem with their back it can stop their developpement or shorten their career.
and those kind of players (tmac, kobe, KG and even Kwame Brown, Eddy Curry, Shawn Kemp or all others) waste their first years in the nba, producing insignificant stats. All those players had to wait at last 3 years to become a leader.