The list of veteran QB isn't all that long for the the Raiders. Their in a tough position because there is no depth the draft at the QB position. If there was, they could draft a guy and let him sit for a year with a veteran at the helm. So do they go out and sign a veteran to be the starter and hope that he pans out long term? Is so, Garcia isn't that guy.
I've said all along that Plummer should be the guy. He has shown that when his head is screwed on right, he can play as good as any QB in the league. He isn't a dominating QB, but he can win any game. Unfortunately he has his moments when he wants to win it on his own and tosses picks all day. I doubt the Broncs would trade him to the Raiders and if they did, it wouldn't be cheap.
That leads me to David Carr. I think he has had enough opportunities to prove himself in Houston and couldn't do it. You have to believe that he has upside and that a change in environment could get him to the level he is capable of. Unfortunately, his situation in Houston isn't that much better in Oakland and it would be hard to believe that this would change.