How is playing in front of smaller crowds than you did in middle school preparing you for the NBA?
Because you are actually playing basketball games, in which you can work on the skills you are trying to improve in a live environment.
Although I agree that practicing against guys like Pierce and Allen can definitely prepare them in some aspects much better, with the schedule the C's have had, there have been very few practices. They would be lucky to get 1-2 full practices a week. When young guys really need to develop skills, that just isn't going to cut it.