Coaches are really, really averse to playing QBs with no starting experience when the games still matter. Garrett, of all people, just isn't going to do it. But you're right. Weeden was still getting guaranteed money from his CLE contract, so it wasn't really a cost issue. They had to have determined he wasn't going to develop further, fast enough, and kept Moore, instead.
Now, reading between the lines from what we do know about the QB position, I think Moore is a young guy they see as a potential QB2 for his career. He's not somebody they're seriously grooming for starting, or we'd be getting tidbits from Jerry or Stephen, or we'd be hearing Romo saying something complementary about him or the receivers would be talking. It's been pretty clear that the team thought Cassel was the one who was likely to have improved here recently or that he was the player they should have moved earlier to possibly add to the roster. Broaddus had some buzz early that suggested Linehan wanted to maybe give Moore a shot in between Cassel and Weeden, but I take that with a grain of salt, too, because it's Broaddus and because they were never going to play Moore when the game still mattered and an experience backup was healthy.
I think they like the guy. I think they think he's got a shot to be a backup, or to maybe push a young developmental player. I don't think they really see him as a legitimate option at QB beyond that.
I don't, either. But it would be nice to have even a young player with moxie back there making better decisions or better throws when it comes to third downs. We absolutely can't win with Cassel playing the way he played yesterday, so if the only other option who knows the system is Moore, I'm all for seeing him even if he's a complete unknown. I can live with the significant risk of extra turnovers if the alternative is to lose, anyway. If the upside is maybe extending a drive here and there and making the games more interesting to watch, then let's do that.