Will you get better at something if you do it for 20 hrs a week or 40-50 hrs? I know it's more common to say that hoops players aren't always students first, but to stay eligible they do need to go to class and do work, which cuts into time devoted to becoming a basketball player.
Maybe some people missed the Grant story where he talks about the extra free time he has in the NBA and how he uses that time in the gym, and how he is able to have a healthier diet. He didn't mention if he had a personal nutritionist, but that is what a lot of players have and certainly beats eating at Goldstein..
A guy like Ennis is probably working a lot with the Suns staff during practices to get familiar with their system, which is better for his long-term future than playing another year of 2-3 zone. Again, the NBA has changed a lot in the last 5 years and playing in the D League is like a baseball player spending some time in AAA.