There's a flipside to the second contract argument that also dictates that a first-round pick come out.
When MD PG Melo Trimble was a frosh, he contemplated coming out as a projected back-half of the 1st round pick. He stayed, and the argument was after his sophomore year, he would be a lottery pick. Well, Trimble's sophomore year exposed weaknesses in his game that caused him to no longer be projected as a first round pick at all. So, he stayed for his junior year, and again, it looks like Trimble won't be a first round pick after this year either, and there is no chance he will ever be a lottery pick. So, when you have a shot for guaranteed seven figures, you take it. Maybe, you can hit it even bigger on your 2nd contract; maybe there is no second contract, but a guaranteed 3 years of salary at $3 million ($1 million per year - which is going to rise as the result of the new CBA) for the last pick of the first round is life changing money. Would never fault anyone for making the choice to head to the NBA if NBA first round money was a certainty.