While you are right to a point, I think it is important to recognize that, even though it is much harder to get into the rotation on a contender than on a rebuilding team for young guys, there are always opportunities. What sets apart those who stick around, and find minutes, and those who stay on the end of bench is whether they take advantage of those opportunites.
Glen Davis, and Leon Powe are two young guys who took advantage of opportunities when they arose on this team, and proved to Doc that they belong on the floor. Gabe Pruitt and JR Giddens did not show that they belonged. Bill Walker started to show some flashes at the end of last year, but did not force Doc's hand, but this season, the opportunity presented itself when he was on the shelf after knee surgery, and the window was pretty much closed by the time he returned. So while it may not be as much to his own doing as guys like Pruitt and Giddens, Walker belongs in that category of a guy who simply was not able to step up when the window was open.
That's a good way to look at it. Of course alot of it, just like anything in life relies on luck and timing. This is also coming from a guy that backed up an all State point guard for 4 years in high school and never had a chance to show my coach I could play. He actually told me, "son, youre going to be a defender and a distributor. i dont want you shooting when you are in there."
If he had paid attention, he would know that I was a GREAT shooter. He sat me if I took a shot within the first couple of minutes of playing, even if I made the shot. Not saying i could have started over the all state player, but my role defintely could have expanded. We also won the state title 3 years in a row when i was there so no one really argued with the coach.
Actually, although I just bored you with my story, as I am writing this I now know why I feel this way about that comment. So my view is a little biased but the coach isnt always right about his assesment of a player and it is unfortunate because it can totally alter the career of a player.