Personally, I've always liked Seymour, and I hope that next year he'll be able to find a team that's a good compromise in terms of the money they're willing to offer, and the type of scheme and surrounding talent that can best feature Sey's skill-set. Unfortunately, the Pats aren't that team on either the salary or scheme fronts.
While the D would certainly be that much better with Seymour in it, I think this is a trade you have be happy with for two reasons. Firstly, in an increasingly pass-oriented NFL, there are fewer snaps a game where you'll want three big linemen like Warren, Wilfork and Seymour all on the field at once. This somewhat diminishes the returns from having the kind of DT depth we've enjoyed for a while.
This leads into the second, which is that the Patriots clearly weren't going to be able to resign both Seymour and Wilfork, and with Wilfork being younger, healthier, and happier in the Pats' system, he's clearly the guy you want to keep. In return for losing Seymour a year earlier, we get some added cap room to help w/ a Wilfork extension, and a likely very early 2011 pick with which to get the next Wilfork/Warren/Seymour type lineman.
Of course, this all depends on our getting Wilfork inked to an extension before he hits FA.
