Yea I saw Cassel miss Moss on a lot of deep balls earlier on more so than later in the year, but he progressed a fair amount as well.
As far as the Thigpen vs. Cassel argument goes, just consider the road they took to get to where they are. Sure Thigpen played at Coastal Carolina, but you don't think Cassel could have started there? What about Thigpen's chances at even being anything than a scout teamer at USC?
Then move on to the NFL. Cassel has spent the last 4 years learning the NFL game in New England and learning from Tom Brady. While Thigpen has two years in KC, learning under the wise tutelage of Dick Curl, Herm Edwards, Damon Huard, and Brody Croyle.
I think Thigpen could be a very good backup QB, but I don't see him being a solid starter ever. Cassel has that upside to him. Is it a risk signing him? Of course, but KC needed a QB and this was as a good if not better option than drafting the unknown.
But you are right that KC has many other needs to address as well, considering how they acquired Cassel and Vrabel, this was a deal to take. The LB core patch work job might hold for a season or two at the most while they continue to procure new players to take over those spots.