I don't understand why someone like Kellen Clemens would be a good veteran backup QB, but Matt Cassel is not. They are the same guy. They've had nearly identical careers. Actually, Cassel has better numbers and started more games. If you are looking for a guy with experience, then there is no reason to reject Cassel over the other guys listed because they are all similar. None of them are better or worse than the other. Pick one and people will complain they should have chose the other. Pick that one and again people will complain they should have chose the other.
What I find funny is the comment about Matt Moore and how the organization was dumb to cut him. At the time Tony Romo had started all of 10 games. They wanted a veteran backup to mentor him and who could be ready in case of an emergency, which is why they went with Brad Johnson. They had to make a tough choice and going with 3 QBs would have been a luxury. A luxury they couldn't afford at the time. Having a veteran QB to mentor Romo is the same thinking that is currently being suggested with Dak. I think they can get by without a veteran backup, but I'm not against it. You still have to find the right guy.