A point is being missed here involving the static vs. the dynamic.
Taking a snapshot in time at the end of preseason, Leinart wasn't as bad as Anderson and Hall in the moment but the coaches must have felt he'd never be good enough for us to win with him.
While it may have been felt that Anderson and Hall were not as good as Leinart in the moment, they had enough upside potential that, in time, they could be developed to a point where we could win with them.
We're still early in that development-curve (and admittedly things look pretty gloomy) but Wiz has already observed that, in his role as a backup QB, Anderson can and is meeting expectations.
With Hall, the issues are - How good can he eventually be? How long will it take for him to get there? And can we afford to wait for that to happen?
The coaches evidently feel that (a) Hall will eventually be good enough. (b) He'll get there quickly enough so that (c) we can afford to be a bit patient.
My own take is that Hall fits a special mold (i.e. short, smart, accurate) but cannot execute the same kind of passing attack that Warner, Leinart or Anderson could. He's short (he often looks like he's throwing out of a man-hole) and has trouble locating open passing lanes (Opposing defenses teams know this, and do everything possible to make him improvise in order to force him into bad decisions). One way to remedy this is to make designed roll-outs and waggles a greater part of our playbook.
(Note - When Hall is forced to improvise under pressure, he has two options - turn left to open up a passing lane or turn right. All opposing defenses have to do is "guess." They may give up a completion 50% of the time, but they'll also crush Hall and force a lot of fumbles the other 50% of the time. If I'm an opposing DC, I like those odds).