With every new preseason game it becomes increasingly apparent that the addition of Harvin to the Seahawks is absolutely stupid. It's so stupid that it's doopid.
Expand please...
Well first of all, he stole that line from Kenneth Arthur....but no matter.
Harvin is the most game changing receiver in the NFL. Forget Calvin Johnson, forget AJ Green, forget whatever Oakland receiver Autumn thinks is going to be good, it's Harvin. Why?
Harvin draws way more attention from the defense than any other receiver. You line up Revis against Calvin, he will follow his every move, you can't do that with Harvin. He moves from the slot to the backfield to out wide. Put him in motion before the snap and suddenly who covers him changes. You now have a 4.4 speed with the fastest acceleration in the game dictating who covers him. Throwing a screen pass, put him on a corner and let him run him over. Throwing over the middle? Put Harvin on a linebacker and watch him try to cover him. It won't end well for the linebacker. He's also the best receiver on hand offs. Look at the Jet sweep plays from the Superbowl. He torches defences with the ball in his hands.
The best part about having Percy Harvin is that he's instantly your most dangerous weapon and opens up the field for everyone else. You going to leave Harvin alone in the middle field or are you going to let the safety hop in and cover him?

Pick your poison.
You could leave two safeties deep, one to cover the fly route, one to help with Harvin, but then you're leaving Marshawn Lynch with 7 guys in the box. Have fun with that one.

Harvin motions out for a screen pass, the defense cheats towards him, Lynch takes it to the house.
For fun, let's split Harvin out wide, run the tight end down the middle on a seam route. Leave Harvin in single coverage or throw a safety overtop?

Either way, you're fu
cked.
Let's break down the offense now.
You have Russell Wilson at QB, the best QB for evading pressure and a QB who's not afraid to chuck it deep. He can beat you with his legs or his arm and is one of the most dangerous play action passers.
You have Marshawn Lynch at RB, a RB who breaks more tackles than anyone in the league and doesn't a s
hit about your existence.
You have Doug Baldwin, one of the most sure handed wide receivers in the league, running clean routes inside and outside.
You have Jermaine Kearse, a smaller WR at 6'1'' but plays big. High-points on the redline like no other.
Paul Richardson, 4.4 speed. Burner.
Luke Willson, a 6'5'' TE with 4.5 speed. Yes, a TE.
Christine Michael, the most explosive RB in the NFL. His SPARQ scores are through the roof.
And now you're adding Percy Harvin to the mix. This is an offense that finished 4th and 7th the past two seasons in DVOA, with the second most explosive plays 10+ yard runs, or 25+ yard passes, behind the Philadelphia Eagles who ran far more plays than the Seahawks.
In 2012, before Adrian Peterson was the MVP, Percy Harvin was the MVP for the first 8 games before he got injured. Ridiculous return skills, and all the traits I mentioned above. He may never touch Calvin's or AJ Green's numbers, but he has a way bigger impact on the offense than either of those two.