And, he's STILL trying. I present to you exhibit A, which suggests that BB has not been as thoroughly "sold" on Cassel as you claim:
http://blogs.nfl.com/2008/09/09/patriots-ahead-of-their-time-again/
Look, I'm not trying to "pigeonhole" anything or anyone. I'm merely pointing out the controversy around BB's decision and floating the idea that BB knowingly took a calculated risk. It's just as possible as your claim that he was dead set on Cassel as the man for the job. In either case, I refer you back to exhibit A.
And, the "best interest of the team" can be interpreted many ways, considering budget parameters earmarked for the ENTIRE quarterback position including Brady and O'Connell, along with how available veteran backups might or might not have fit the Patriots system.
This doesn't make sense. It very well could be that Brady's proven durability over 110-plus consecutive starts plus the pool of available veteran backup candidates were factors in BB's decision to retain Cassel as No. 1 backup. In any case, BB is fully accountable.
So, how do you gauge if a veteran QB with lots of experience is good or not? A lot of his success depends on his supporting cast, and coaching. For a player to even get significant game experience, he must be pretty good to begin with, wouldn't you agree? But don't take my word for it on the experience issue, give a listen to what Landry had to say.
Once again, I'm backing Cassel and BB on this and am not claiming BB made the wrong choice in not having a veteran backup at the ready to lead a proven Super Bowl contender. I absolutely want Cassel to succeed. I'm just saying, it's not as black-and-white a situation as you suggest and is open to question for valid reasons.
You say Cassel is starting by design, which is possible. I'm saying he just as well could be starting by default.