The addition of Seymour is obviously going to upgrade their D-Line, but the price they paid is over the top. The guy is no longer in his prime, although he is still a good player. It was a nice addition and a decent move for the raiders THIS YEAR, but next year and beyond it might not help them much. Like others have said, he will probably walk after this season and the raiders basically gave up a top pick for a one-year stint with a past-his-prime Richard Seymour.
Obviously this is going to make their team a little better this year without any sort of drawback, but the drawback will come next April. Maybe Al Davis figured out he blows draft picks anyway and has just decided to trade them away.