One, signing Matt, tagged or not, would tie up a disproportionate amount of Cap room at QB, when we have holes to fill and when we might want to cut early deals with players whose contracts are up. I don't think tying that much money up at QB is in the best interests of the Franchise.
Two, arguing that Matt's value would go up is an assumption but not a certainty. Also, given the risk of career-ending or career-limiting injury every game, it makes no economic sense for Matt Cassell to do anything other than ink a multi-year deal ASAP, if the Pats don't tag him. So, I don't think this is in Matt's best interest.
Three, in an ideal world, Brady and Cassel would be singing Kumbya. But these are competitive guys and, at some point, there would be strains on relationships and possibly performance, no matter how great BB's coaching and no matter how well-intentioned everyone was. So, I don't think it's in Brady's best interest.
(Suggestion: someone else put in "zzzzzzzzzzzz,"which I won't do, but you could try to boil your points down or limit them to a few rather than many. I'm sure you wrote some things that I skimmed or missed because it was just tooooooooo long. There are very few folks out here who have earned the right to post something real long and expect the rest of us to read it all.)