I am a bit late to the party here, but this was a great move by the Pats. For one thing, you cannot put a price on a first round pick from the Raiders. I would be shocked if that is not in the top 15 picks, if not top 10.
But more importantly, as good as Seymour is, this team did not NEED him. He has the ability to dominate the game as much as any defensive player in the league. But the key word there is ability. The consistency was not there. He always seemed to be injured, or just not playing up to his potential for whatever reason.
Now don't get me wrong, it will hurt them not to have him. But its not like they could fully count on him anyways. Besides, with the struggles this team has had defending the pass, the fact that they can put more of a passrusher in there for him might help to mitigate what they lose in the run defense.
Ultimately, I think it was clear that he was gone next year anyways. He simply has not shown the consistency needed to get the kind of contract he is going to be wanting next season. So for them to be able to get not only a first round pick, but a first round pick from the Raiders for him now is an absolute coup.
Now, hopefully this means they are committed to resigning Wilfork, because if they let both of them go, then they will be in trouble.