I think some coaches/GMs get caught up in the guys potential and have a blind eye to a players negatives. Cassel looks like, and sometimes plays like, a good NFL QB but he doesn't do it on a consistant basis. Coaches and GMs think that if they stay with the player, or get the player more talent araound them, that the player will somehow be the player that he is on the good occasions more often than not. Cassel has shown that he is an inconsistant player all of the time. It is past time for the GM to realize this and move on.
Never be satisfied with what you achieve, because it all pales in comparison with what you are capable of doing in the future.