Sora can say all the depressing things he wants, but that doesn't mean he is developed at all. Sora is EXACTLY the same as he was at the beginning, literally nothing has changed. Even if Nomura wanted him to stay that way, that isn't a good character at all.
Good characters have to go through things, understand their mistakes, and it has to change them in some way. The most throbbing example of this is Riku, obviously. Hell within Days, Roxas got developed more than Sora and we haven't even had that much time with him. When he says that the darkness got to him too, that changes nothing, we couldn't tell that the darkness effected him, and the darkness has been known to affect someones actions.
Sora got developed an assload in COM, with all the nonsense going on with Repliku and Namine etc We got to see his character grow more, but Nomura wanted his memory wiped for some reason, throwing all of that out the window. So instead of maturing, by the beginning of KH2, Sora is back to his childlike mentality. How I see it, Sora is only here to move the story forward and save TAV. That is the only role I see for him right now.