Well, Cable does have a point. Seymour is worth a heck of a lot more than Darrious Heyward-Bey, JaMarcus Russell, Darren McFadden, Michael Huff, Fabien Washington, Robert Gallery, Derrick Gibson, and Phillip Buchanon. Seriously, the only first round pick that the Raider made over the last decade that wasn't at least a minor bust was Nnamdi Asomugha. Ok, Sebastian Janokowski wasn;t a bust, but he was a kicker taken in the first round.
Odds were that Al Davis was going to blow the first round pick in 2011 anyway. So yes, Seymour is worth more than the punter or the sixth best WR in the draft with no WRs taken at that point that Al Davis would have picked. For any other team, taking a DE who gave you 47 tackles and 4 sacks and is now 30 for a future first rounder (even two years out) was a bad pick.
The only way them trading for Seymour was a good pick for them was if Seymour was the final piece in their Super Bowl puzzle and he made them a legitimate contender. Right now, there is no guarantee that Seymour will even be a Raider when the Pats use that first round pick.
You know, the Raiders might have been able to get Seymour this offseason without any compensation if they didn't make the trade. The Pats probably still would have franchised Wilfork and alowed Seymour to go to free agency where the Raiders could have offered him the best deal to come to them. Granted doubtful he would have willingly gone to the Raiders.