And the reason you don't give up on JA is simple - you don't yet know what he is.
Sure Green and Neal have passed JA up in the rotation for now. However, it should be obvious to even the most casual observer that RJ will be off this roster next year. All of which is why you don't give up on JA just yet - at least not for another year. Why compromise your depth at the wing spots by entering next season with 2 open spots on the perimeter, instead of one? That makes no sense.
Now, if JA fails to impress at the start of next season, then he could easily be used for trade fodder by the Feb '13 trade deadline.
For now, the Spurs can ill-afford to start developing a pattern of allowing young, developing players to walk before they know what they are. I trust the Spurs have not learned a valuable lesson from their short-sighted decision in allowing the young, French big to walk away and become a crucial part of the rotation in Dallas.